Is the purpose of this get together for your friends and family to have fun together? I think the purpose of any get together is to make your guests happy rather than only to make it as convenient as possible for yourself. Since she did tell you in advance, I think it would be nice gesture to accommodate her as a guest, not to say effectively, “suck it up”.
I never ask my guests to bring anything and try to accommodate everyone… That said, this is a relative. I would have replied “thanks for letting us know - we will keep this in mind for future events” and left everything as is. Did not care to give me an advance notice? Too bad so sad. 25 years is old enough to appreciate the value of common courtesy. It sounds like this event requires elaborate planning at a very busy time, so this out of the blue announcement will send the hosts scrambling… If everyone bends over backwards and keeps accommodating her lack of etiquette, I doubt she will learn anything.
Re the original question, no, the statement itself doesn’t seem particularly rude although your husband’s sister chiming in absolutely does seem rude! How did she get involved anyway?!
OP planned, ordered, and presumably prepaid for a wide variety of food that as far as she knew would meet the needs of guests. We’re talking about stopping on the way home from all day services and likely 24 hours of fasting, bracing the crowds at the store to pick up the trays, and rushing home to get the food on the table for very hungry guests. I can’t imagine making an extra stop or purchase under the circumstances.
I would ask the sister to bring something for the cousin if she’s so concerned. 
Really what this sort of amounts to OP is…are you willing to be the bigger person here? To make the situation as cordial as possible the night of your event - an event I’m sure you want to be a success - for everyone?
If you feel she was wrong and that you should not have to accommodate her in your heart - well, then you know it . But find 15 minutes the day or two before the party to stop at the grocery store and be the bigger person and make things work.
'I would ask the sister to bring something for the cousin if she’s so concerned."
First met me say that our families are really not into drama. Nobody holds a grudge. Sister wasn’t like " how dare you be rude" she was more like " hmmm that seems sort of rude" and tone is hard to convey here but it was said in a friendly manner.
BUT update: Sister tried to help and sent us the following copy of her text with cousin
Sister: Hey [Cousin] I hear you’re a vegan now, don’t worry I’m thinking of bringing a Quinoa salad
Cousin: Oh thanks by salad do you mean it will be cold
S: yes
C: thanks but I only like it hot. I also love meatless chili
S: you are welcome to bring that to Maya’s I’m sure
C: I can’t. I’ll be at services right before
S: yes we all will. Well looking forward to seeing you!
S: (to us) lol!
Cousin sure sounds like a piece of work but hooray for sister for trying!
Again, I guess choosing whether or not to be gracious and accommodating or not depends on how much to heart you take the significance of the holy days. What SHE does or says wouldn’t matter to me; my attitude and behavior would. YMMV
Now THAT is ridiculous. If the relative is that picky she can bring her own friggin’ meatless chili. Not exactly “break the fast” food. I would politely encourage her to bring something she likes to eat.
I have mentioned before that I have hosted holiday dinners where one family had a vegetarian, a kosher and a gluten free member (all different people). We had to have signs color coded in front of the foods to indicate what was kosher what was gluten-free what was vegetarian etc. The wife asked if I would be getting a kosher turkey (it was Thanksgiving) but that would have required getting a bird that was over 20 pounds which (A) I couldn’t find kosher and (B) if I could it would’ve been cost prohibitive. I let her know that unfortunately I could not find a large enough kosher turkey but they were welcome to bring whatever they would like. They brought a Tofurkey and a kosher turkey breast.
Wow, @maya54! That cousin is annoyingly picky! You a kind person to include that cousin and sis was a sport to try to help out. I’m sure whatever you do will be just fine and more thoughtful than I’m feeling just reading this thread.
There will be vegetables at the break the fast…and bread. They are not trying to starve this cousin.
She expects to have others prepare a specific hot meal for her? Sheesh. Maybe someone can get a few cans of chili and heat them in the microwave.
Oh dear. Throw the graciously offered quinoa salad in the microwave and call it a day. Again, texting misses nuances. Pick up the phone and talk to cousin about concerns.
I misread the original post and thought it was DH’s niece and that he was called out by her mother. I don’t think I’m alone in that. But DH, his cousin and his sister are the players, correct? So sort of peers even if there’s a big age gap?
I have a cousin with many idiosyncrasies and no filter (mentioned in post #64). Over the years I’ve found it easiest to call and politely work out her issues before an event. There are many moving parts in any family puzzle. My goal is to keep our events drama free. Doesn’t always work, but I’m getting better over the years (now decades).
“Pick up the phone and talk to cousin about concerns.”
She’s 25 and like some millenials don’t seen to ever pick up a phone ( or listen to voice mail)
Wait, I forgot the best story. While she’s our cousin not our niece the family tries to look out for her closely because her parents live far away. Brother in law wanted to see if she wanted tickets to a performance but it was hard to explain what exactly it was and the logistics were going to be complicated. So he called her many times and left message but she never picked up,or returned calls. So BIL texted her " can you call me when you get a chance" And she called and was like " I was so scared when you asked my to call, are my parents ok?"
Your cousin sounds like a drama queen when it comes to food. If she wants a specific dish, then she can bring it herself. Has she never heard of crockpots? She could bring her meatless chili or her hot quinoa dish to your house before the services…plug in the crockpot and away you go. Thousands of people do this every year for various religious holiday events that involve attending a religious service, then having a big meal at somebody’s home.
Or yeah, you could make the quinoa thing and nuke it in the microwave.
Strike anyone that we’re noting the limits of texting, but the only way we all commuicate here is the equivalent? No tone of voice, no facial expressions, little nuance.
@lookingforward Lol. See my story in post 92
The problem for me is that she is an invited as a guest of relatives I assume she knows well enough to know they are NOT vegan, unlikely to be serving a vegan menu, and will also be at services all day, and then drops it on them late in the game that she’s now vegan.
The manner and timing along with not offering to bring anything comes across as saying she expects them to provide foods she will eat. I’d personally find that a bit presumptuous and rude. I don’t know if I’d respond the way the OP’s husband did, but I know that’s what I’d want to do.
Unbelievable. After seeing this, I think the most I’d do is hand her a jar of peanut butter and ask her if she wants me to microwave it because I know she only likes things hot. 
Ha, Maya54, had missed it. I liked 84, too.
One of mine doesn’t regularly read non-work emails I send (apparently, friends are on her for this, too.)
“After seeing this, I think the most I’d do is hand her a jar of peanut butter and ask her if she wants me to microwave it because I know she only likes things hot”
Lol. We actually have relatives/ guests with life threatening peanut and nut allergies! That I go out of way to accommodate and will anytime. BIL who has the allergies is VERY laid back about it but it scares me so much! We once had to divert from a dinner out to the ER because DH ordered fish with macadamia nuts not realizing that this nut has a powdery coating that spreads thru the air. BIL had to practically be forced to go to the ER he was" sure he’d be fine" even though he was having a reaction and his epi pen didn’t seem to help. After we all yelled at him ( lovingly ) he agreed to go.
I’ve adopted an almost-vegetarian diet–not out of any deep philosophical convictions or health needs, but just because I feel good and it helps me lose weight. My system is to control what I cook at home, and not worry too much about what I eat out. I figure it balances out in the end.
The funny thing is that both my mother and my mother-in-law have been very supportive of my fitness and weight loss goals. So, they will say things to others on my behalf–“oh, Fran won’t eat that.”
Several times lately, I’ve had relatives tell me that they’ve cooked something special for me–but I hadn’t asked for it! One of The Grannies said they must do it for Fran. I’ve asked The Grannies not to make special demands for me, but they are both so eager to be helpful. But then people are annoyed at ME for being picky!
This is just to say that this mix of families and food and expectations can have MANY different permutations.