It’s unclear, but have you actually explained to your niece that this is required, not optional and showed her how/why you know that? If you’ve done that, she’s old enough to decide how important it is to her and understand how her choice impacts whatever consequences she receives.
Oh my MIL and I have both stressed that it is mandatory and not optional. Including pointing out where in the multip-page set of instructions where it says it in big font, bold letters etc. I talked to her about how it would be embarrassing to be one of a few - if not the only - child that showed up not doing the work, not to mention that it could give the teacher a negative impression of her that may be touch to overcome. I also reinforced that she needs to read the book and do the work in order to be prepared for the “presentation during first three days of school” mentioned in the instructions.
I also talked to her about how waiting until the last minute would be painful and to try and do a little each day up until their vacation, so that she wasn’t scrambling trying to do this in the weekend before school starts (early August here). She smiled and nodded and agreed but who knows what really sinks in.
I loved uniforms. My kids loved uniforms. They wore them k-5, then again in 9th grade. One of them said, along about 11th grade “Uniforms were so much easier to deal with than dress code rules.”
Normally what they are talking about now is a polo shirt (in one of 2-3 colors) and khaki or navy shorts/pants/skort. It is so not worth making a big deal out of it.
I am not into school uniforms at all so took that into consideration with schools for our children I am unclear and sorry if I missed it - do you have children of your own @PennyS ? Have the parents asked for help from you and your MIL with any of this?
I know this is not the original quandary, but you posted:
This attitude is truly problematic, as school and home need to be a team and support one another. Your vignette immediately brought a memory back to me. I was a teacher in a multi-age grade 1/2/3 class a long time ago. I remember a first grader who was a discipline problem. In my class, there were various levels of “consequences,” for discipline infractions. At the highest or strongest level, after going through the more minor level of consequences, was a note home to their parents, which most kids wanted to avoid, for feel of really being in trouble. I remember one time that all the more minor consequences for this boy did not affect him and so I had to send a note home to his mother. She wrote a scathing letter back to me and I don’t recall all the details now except the ending which was that she doesn’t send him to me when she has a problem with him and so I should not send him to her when I have a problem with him! I remember showing the note to my principal (and I saved it for years, as it was most unusual). The principal and I decided that we’d get no support at home and so when the student’s behavior warranted a big consequence, he’d have to go to the principal and forget about involving the parent at all.
PS, I agree with the others that your sister is sabotaging her D at this new school by not having her wear a uniform and by not requiring her to do the assignment (even if she does a crappy job on the assignment). I feel for this child. Also, a low grade is better than a zero. If her writing is poor, or if the length is too short, the teacher will see her level of skill, which is beneficial.
If this is your SIL and BIL, aren’t your husband and MIL directly related to one of them? If your husband and the person’s own mother can’t get through to them, I don’t see how you will.
Your SIL has made it clear that she doesn’t want to get involved in her own child’s education. I’m not sure there’s much you can do about that. As your niece gets older you can provide emotional support or a sounding board, if she searches you out, but for now it seems like you’ve done all you can by walking her through a couple of the reports and letting her know the school considers them mandatory. She’ll either do them or not.
Also, I think it is pretty unusual for a parent to say to a child that they don’t have to do their homework.
As well, not wearing a uniform is not going to last too long. It’s purely dumb.
why not ask the 'rents to e-mail the teachers to confirm that it is indeed ‘optional’. If the teacher says ‘no, its not optional’ and the 'rents continue to blow it off, that’s on them. (not a great way to start at a new school, but thier vacation may be more important to them.)
My husband’s sibling is the parent of my niece, so that is why I keep referring to them as BIL/SIL - as that is what they are to me. 
My niece is the one who asked for help once MIL pointed out the assignment wasn’t optional and stated she was overwhelmed, didn’t know where to start, and had never done this type of assignment before. I offered to help when I saw BIL/SIL in person recently and they were fine with it.
MIL did tell me she encouraged the parents to confirm with the school that it was optional but they said they were too busy and she could call if she wanted. She didn’t think it was her place so has not contacted the school.
If I had a good relationship with the child, I would offer to take her to Target as a treat to buy some clothes, that just happen to the be the uniform. She could then decide to wear it on the first day or not. As for the assignment, offer to help her or have her do what she can. I would worry that showing up both not in uniform and not having the assignment on the first day may be really tough on her. At least with the uniform, she won’t stick out as much. But if she won’t do it or her parents aren’t making her, not a lot you can do about it.
Have the parents request and read dress code.
The BIL/SIL know perfectly well that it is not optional, it is in bold in the instructions. To me it sounds like they are (childishly) resistant to a school with rigorous academics and stricter rules. They may get on board once the school year starts, or they may not. The OP isn’t going to be there every day to make sure the kid’s homework gets done (which is probably going to be required). So the kid and parents are going to have to sink or swim. Maybe the parents like the IDEA of a rigorous school, but have no intention of actually inconveniencing themselves to make it work. Time will tell if they adjust their attitude or not, but I see little that the OP can do. (I say this as a parent who LOATHED school uniforms, and still see it mostly as a way to control girls’ sexuality and a costly item for parents… but we did it).
Your BIL/SIL are idiots. I wouldn’t worry about the effect on the English grade. It won’t matter. The uniform thing is silly.
The niece is going to 6th grade. At that age she is capable of figuring out what’s important for school. She may not get good grades on her first assignment, but if she cares and wants to stay at that school she will figure out what’s mandatory and what is not. Yes, it would be more helpful she has her parents’ support, but many students (with not so educated parents or first generation parents) do figure it out.
If it is my niece, I would take her out for lunch and let her know I am there for her. If she needs help with homework, getting supplies for school (in case her parents wouldn’t get them for her) she could always come to me. As far as the uniform, I would get the grandma to buy the uniform and send it to the niece. The parents are not going to get mad at a meddling grandparent.
She can complete the rest of the one page summaries herself quickly (or not, or badly - she won’t be the only one). Offer your help for the week before school starts, just for the big paper (even if the short papers are bad let it be).
The uniform issue will sort itself when the parents are called at work and summoned to the school because of the dress code. (It sounds more like a dress code than uniform).
When you say this: “Money is not an issue regarding the uniforms. Plus it is colors vs a specific purchase from the school, they could get the clothing at Target or Old Navy. Plus I have offered as has my MIL. Why they would deliberately make this difficult is beyond me.”
Perhaps, since they could get it at Target or Old Navy, they’ve simply put it on a back burner? Or even placed the order without your knowledge?
This isn’t about the kid. This is about the parents and the school. It’s not the school’s exclusive job to educate kids – it can expect support from its kids’ families – but it is the school’s job, not the OP’s, to educate the parents of its kids as to what the school requires. I think the OP, having already crossed any number of boundaries that people are usually hesitant to cross in families, ought to step back and let the school do its job. I trust that the teachers and school will have encountered this kind of problem before and know how to handle it without making the situation worse for the student. There will almost certainly be some trauma, and it will almost certainly be short-lived, one way or another.
You are a good aunt and from everything I have read, it appears that you have gone above and beyond and have done everything possible to help. Despite that, her parents are unwilling to play by the rules of the new school. I really feel for your niece, as I am sure that she will feel upset and/or humiliated being a new student and showing up not in dress code and without her summer assignment. I know I would be. As a former board member of a private independent school, I am well aware that private and charter schools can make their own rules and the expectation is that the student (and parents) will adhere to these rules, whether or not they are in full agreement with them. If you don’t, then they have the option of finding another school - including the public school - where they don’t have rules that you don’t agree with. Bottom line is that this school is not a good match for her family.
“Bottom line is that this school is not a good match for her family.”
Not now it isn’t. But there’s a chance this could still have a positive outcome for the niece. The niece might have an awful first day and realize there are some things she can do to improve her situation, like pay close attention to assignment instructions. Without knowing the niece, there’s no way to tell if this will be one of those needless, crushing experiences that just discourage her and which she’d not at a point where she can learn and benefit or… if this will be a turning point where she realizes her parents aren’t helpful and it’s up to her to make sure she has what she needs done.
At that age they do not like to be embarrassed. She may just raise up to the occasion if she knows the alternative is going to a big public school.