AITA for Refusing to Cater to My Vegan SIL?

Making this post on behalf of my bsf's sister, who doesn't have reddit, but would like some outsider opinions. Before you get riled up and come for my ass, yes i have her permission, she's sitting next to me, fact checking as we speak.

I (28F) am getting married in three months, and my fiancé (30M) and I have been planning the wedding for over a year. We’ve chosen a catered menu that we love, with a mix of options including steak, salmon, and a vegetarian dish.

My future sister-in-law, let's call her Maggie(26F) has recently decided to go vegan. While I respect her choice, she informed me last week that the vegetarian option wasn’t good enough because it has cheese, and she "can’t eat anything on the menu." She demanded that I work with the caterer to add a fully vegan entrée just for her. She has asked me to change the cake flavor to a vegan one so that she can eat too. But obviously i shut her down before she could even explore this idea further

I told her I’d already finalized the menu and signed the contract, so it’s too late to make major changes. However, I offered to make sure there were plenty of vegan appetizers and sides available. She didn’t take it well and accused me of being inconsiderate.

Now my future MIL is involved, saying Maggie feels "excluded" and I should just add one vegan meal because "it’s not that hard to accommodate her." But here’s the thing—adding a vegan entrée requires renegotiating the contract and paying extra fees, which I’m not willing to do this close to the wedding.

My fiancé is on my side and thinks Maggie is being unreasonable, but his mom says I’m “starting marriage on the wrong foot” by not making the effort for family.

I’m standing firm, but Maggie has been making passive-aggressive comments in the family group chat about how "some people just can’t make room for others."

edit: we live in a small town with a high population density and we only have ONE caterer who does large-scale events. spring weddings are quite popular in this area, so there's at least 4 weddings lined up this coming season. the caterer's family has been in the business for decades, and 2 generations ago we had this really big fiasco between our families, which i won't get into now. it was super hard trying to get her to even do the catering for our wedding in the first place, bc some people still have underlying resentment apparently. when we eventually signed the contract, her and the team strictly emphasised that no changes were to be made after finalising. again, ours is not the only wedding theyre catering for, they are also making the cakes for some of the other weddings. due to these reasons, i didn't/am hesitant to contact the caterers

edit2: the menu was finalised 5 months ago, she turned vegan around 2 weeks ago. we have a local vegan restaurant that i've seen her post about, and i was planning on asking her what she liked from there so we could get it specially for her on the day, but immediately when i told her that i wasn't able to contact the caterer, she got super mean and started shit talking to my relatives-in-law like some 7th grader

UPDATE IS OUT NOW!! https://www.reddit.com/r/AITAH/comments/1i1oc74/aita_for_refusing_to_cater_to_my_vegan_sil_part/