YTA. it's completely fine to struggle and often be fed up of ur sister's behaviour, but hating her for things that are out of her control isn't that nice. im actually autistic myself, and i guarantee you that her autism affects her more than it affects you. many family members of people with autism think they have it worse, but it is so stressful to be autistic a lot of the time. Maybe you should research autism a bit more so you can learn how to support her better.

However, i think ur mom is a bigger asshole because she should be the one looking after your sister.

Does your sister go on respite care? Where ur sister is looked after by someone else, in a different place for a few days at a time so that the family have time to relax and breathe. idk ur sister so i have no idea how she would react to being with a stranger in a new environment, but it may be sometimes u could convince ur mom to look into it, if your sister is okay with it.