I don’t yet have children in law. My history is that both of my own parents called their in-laws “mom” and "dad, which is what I thought was pretty standard. Each set of my grandparents loved their SILs and DILs as if they were their own; I thought that was just the way it goes and that all of those MIL jokes were just that - jokes. So when I married, I started calling in-laws Mom and Dad (I barely knew them when we married). FIL, whom I liked, has passed away but I have never had a close relationship with MIL and she is one of the few people I can say that I really cannot tolerate well. And every time that I say “Mom” is sticks in my throat and really is unpleasant, but I feel like it’s too late to change now after over 25 years of marriage. So, I pretty much just avoid calling her anything at all, unless absolutely necessary. For some reason that always reminds me of the Seinfeld episode where he can’t remember the name of the woman he is dating, so he just avoids saying anything: “Hey there,…you” 
We haven’t had a formal conversation with our DIL, but I sign emails to her with my first name, and I think she is picking up on that. I wouldn’t want her to call me mom. I’m not her mother, and I remember how incredibly uncomfortable I was when first married and had to call my MIL “mom.” This really bothered me. She wasn’t my mother and I didn’t have particularly warm feelings for her. Now I call her “mom” (she’s 93 and my mother passed away over 10 years ago) but it doesn’t feel like anything to me so it doesn’t bother me.
I’ve been married over 20 years, and my MIL has never given me leave to even call her by the shortened version of her name that most of her friends use. She refers to herself in the more formal version in cards that she sends me, etc.
My SIL and BIL barely speak to me, and almost never even send their nieces a card or speak to them. I guess they are still mad my DH moved across country to live with me, but, seems like they would be over it after seeing him in a happy marriage!! :-/
I can’t imagine calling anyone else mom except my mother. I grew up calling all adults Mr and Mrs. Here in the south my kids grew up calling adults Miss first name, except teachers.
I’d rather call my in laws Mr or Mrs than mom and dad. We each called our in laws by their first names.
My sister never had a name for her mother in law so once the kids were born she called her what the kids called her.
My mom and her mother were both known to the grandkids and all their friends as a nickname… a la nana Mary.
I think it all depends on the relationship and the people involved. Some in-laws are very loving and parent-like, so calling them mom and dad isn’t a big deal.
I called both my MIL’s (former and current) by their first names. It didn’t have anything to do with the quality of the relationship. I’ve actually never thought anything about it. I guess I should have asked (?).
I wouldn’t mind if my future DIL’s (should I ever be so blessed) called me by my first name. Now if I become a grandma, they can call me grandma or whatever the grandma term is (that would be nice anyway). That’s what we have done.
I used “Mr. B” and “Mrs. B” literally, when I addressed my in-laws for ten years until the kids came along and since then it’s been “Mr. B” and “grandma.” My kids SO’s address us as Mr and Mrs. Last name and my kids reciprocate to their parents. In our culture this is very normal, it would be considered disrespectful to address an elder as an equal. And even though the kids’ SO come from different backgrounds, one Asian and the other east coast families, they apparently have the same perspectives. I guess it would make me feel odd if they used my first name after they married but I’ll cross that bridge…
My kids spouses and SOs call me by my first name. I introduced myself that way. I didn’t meet the spouses until the kids were out of college.
I wouldn’t expect them to call me Mom, they each have a Mom. But they do call the grandparents Grandma and Grandpa; one typically has multiple Grands.
I was a still a teen when H and I got together and parents of friends were always Mr/Mrs. After some time, I was able to call my in laws Ma and Da, as my husband did, but my parents were Mom and Dad to us. How lucky we had an easy way to differentiate.
I introduced myself as my first name to my son’s significant SO. She said it’s hard for her, but she calls me by my name. Same situation with husband. If it’s a new relationship, I’m fine with Mrs. and Mr. I would prefer not to be Mrs to DILs, when we get to that point.
I would feel awful if my kids’ spouses called me Mrs. ______. If they don’t want to call me “mom” or by my first name, maybe we’ll come up with something else. But, Mrs. _ ?? yuck!!! I’d hate that.
Maybe we all can come up with an alternate name? 
Grandmas have a variety of names, why not MILs?
I have friends who instruct their kids to call me “Auntie [first name].” I don’t particularly care for that, but I figure it must be a cultural thing.
@mom2collegekids perspective is everything lol. I mean my in laws are fine with me using the formal salutation and that’s how I grew up so it’s “normal” sounding to me and not awful. I guess when it’s time for kids inlaw I would like them to just use mom and dad, but we aren’t outwardly affectionate so I’m not sure we would inspire that level of affection 
I had not considered this topic. First names were used with my in laws, but even their kids called them by their first names rather than Mom and Dad. When with Japanese and Korean family, in Asia, I have been assigned a name based on the relationship of the youngest child to me, which is usually as the auntie. Endearing!
Might there being something similar in the Chinese speaking world? Future DIL and SIL are both Mandarin speakers, and will be around them in Chinese speaking environments.
My father always called my mother’s parents by their first names, so that is the model I grew up with.
My husband and I both called our in-laws by their first names. No one objected. I fully expect to be called by my first name by any future SIL.
In our culture, we rarely call anyone older than us by 1st name. Auntie and Uncle are common, as well as many different cultural names for grandma and grandpa. H didn’t call my folks anything until our S was born and then we called them last name grandma and last name grandpa. It worked great and most of the other grandkids do as well. A few of my inlaws call my folks mom and dad, but most of us just say grandma and grandpa.
We don’t yet have inlaws. My sis is the only one who does. Her older SIL calls them mom and dad. Not sure whether the other SIL does as well. They all get along nicely.
It’s a little old-fashioned, but I thought my mother’s solution of calling her in-laws, Mother B and Father B was a good direction. I think a lot of people end up calling their in-laws whatever the children use for Grandma and Grandpa.
I like the “Mr. B” or “Mother B” solution. If you were raised in the south, there is no good solution. In my deepest gut, I would prefer to be called “Mr. Surname,” but I recognize that this is pretty old-fashioned.
This may be even more challenging for my wife and me, because our adult children, for reasons I can’t really explain, still call us “Mommy” and “Daddy.” One of them looks pretty close to getting married, so we’ll see what happens.
Oh well - in some areas of the south I’d be mama to the kids-in-law, and then big mama once I become a grandmother…
Our DIL calls us Mom and Dad, just like our sons do. She asked what to call us, and I told her whatever she pleased. However, she calls her parents Mom and Dad in their native language, and our son calls them that as well. Easy-peasy solution.
The other partners call us by our first names, as do all our kids’ friends. The local southern kids call me, “Miss MyFirstName” which is the custom here.
I kind of like the idea of being “Big Mama” but since both my kids’ grandmothers rejected that title, it’s not something they are familiar with and I doubt it’ll fly. I imagine I’ll be grandmama.
@Hunt - Sometimes I think kids get stuck calling you something and just don’t think about it. While my 14 & 17 year olds call me mom for some reason there are times that my 21 year still calls me “mommy”.
When my kids think they need to be formal or serious or explain the world to me in some way, they call me Mother. Kind of like if I called them by their full names, to get their attention, when they were little boys. If my DIL ever calls me Mother, it will make me very nervous.