Please read!!! My grandmother is being mean!?

Okay, so tomorrow(thursday) I am going to work with my grandmother. (its -bring ur grandchild or son or daughter to work day) and my friend emma was going to come. she is now sick, and she cant. my little brother, Mason, is autistic. (a learning disability) I texted my grandmother and said mason is coming instead of emma. she texted back-not a good idea. my little bro is 10. he is VERY smart, (and his autisim isnt that bad~not at all really, just extra help in school, not talking till he was 4, etc) so anyway, i called her and said "umm....but you didnt have a problem when emma was coming, so y now that mason is coming u do?" and she was like "because, it is all girls your age(13) no boys this year." im thinkin', ya know, not EVERYone that works there's kid is 13. i highly doubt that. so anyway, it continued, and she kept on being sexist and rude. so, my mom isnt letting him go b cuz her and i think my (mimi) wont let him come or is embarrassed to bring him b cuz hes autistic. what do i do? sorry 4 typos, im on a phone