My family is doesn't want my boyfriend'a daughter at the cook out...

My boyfriends daughter is deaf in one ear and has autism. She is 16 but still wets her bed nightly and will.have an accident during the day.if she doesn't make it in time so she wears pull ups. She has the mind of a 5 year old. My brother likes her she and actually really enjoys her company. My family is having a huge get together cook out in Arkansas and we were all gonna stay at my mimi's. But my mom called me to tell me that nobody in the family wants his daughter there because she will ruin it and her mother doesn't want a grown women pissing on her guest bed. I told her if his daughter can't go home of us will go and my brother asked if he could come over instead of going to the cook out. But my mom said no. His daughter was so excited for the cook out and is so sad and doesn't understand why we aren't going... What makes no sense is my brother is 10 months older than.her and he still wets HIS bed but they don't mind. Only difference is my brother doesn't where anything for it so he just pees the bed(except at my house. My brother knows the rules at my house is he has to wear protection) so their reasoning is stupid and seems they are trying to hide being abliest.