AITA for refusing to let my grandchild be a flower girl at my daughter's wedding?

I (40 F) have been married to my husband for almost 20 years. We have 3 kids; boy/girl twins who are 17, and a younger daughter who recently turned 16. Our son and his fiancée are getting married in the summer in our backyard.<br><br>My son and his fiancée are having a relatively small wedding. They've talked about it with us, and they're not having any bridesmaids or groomsmen. For the ceremony, they just want to have the flower girls and ring bearers. Our two youngest kids, along with his fiancée's sister's kids, will be participating.<br><br>Our daughter has significant special needs. Physically, mentally, and developmentally. She is essentially functioning at a 4-5 year old level, and she is also physically disabled, needing to walk with assistance. She is a very sweet girl, always smiling, and she knows people generally love her.<br><br>I told my son and his fiancée that we think it would be really hard for her to participate, and we think it's a bad idea. She doesn't like big crowds, and the wedding will be an absolute sensory overload for her. I don't think she could handle walking alone down the aisle, and if someone had to help her, it kind of defeats the point. My husband and I would both be preoccupied with our roles as hosts, so we wouldn't be able to be there to help. I told them she could be involved in other ways if they wanted, like handing out programs or favors.<br><br>My son and his fiancée completely understood. It actually took us bringing it up for them to realize why it might not be a good fit. <br><br>His fiancée's sister, however, was not very happy about this. She says her kids (who will be participating), are very close to our daughter, and think of her as a cousin. She wanted them to all be involved together. We told her no, and she called me an asshole because I'm trying to 'hide' our daughter from the rest of the family. I cried for a while after because I felt like such a terrible parent.<br><br>AITA?