It’s a REALLY big, scary problem.
I’ve used this curriculum/book with students to teach them about their bodies/bodily autonomy/safety. It’s very simple and covers many health issues. Depending on your sister’s age, it might be worth it to look into (for $11 USD) as it’s very cheap.
Holy crap, I hate this woman! I can never understand this. My late MIL thought my DS was defective (which is why she was HATEFUL Helga), but I think my DS is a great joy! Autism does not mean defective, just different. My DS is free to be himself...funny, smart and sweet. DS has cancelled Halloween because he thinks trick or treaters will scare new kitty! How can you not love someone so sweet?
Give SB a fist-bump from me and DS. It's awesome that he is learning new skills. There is a whole new world out there for him, in spite of that disgraceful woman! I hope awful stepmother meets a painful end. Or suffers from a disfiguring disability. Only kidding...not.
Edit: this book was very helpful teaching my son some life skills...
It might help Grandma.