SIL’s hair in appropriated style? *READ FIRST*

My SIL is special needs and I care for her. One of the things I have paired myself with is doing her hair. Anyways, she’s obsessed with “black people” hair styles. She has autism and FAS. She wants her hair in corn rows and stuff and my MIL does it and she’ll wear hoop earrings “like black people” and listen to rap “like black people”. I’m appalled that she talks like that and mom doesn’t blink an eye but it’s not particularly my place to parent her. I told her that I refuse to cornrow her hair. I explained that its not a style for her hair and even if it was, it would ruin it. I’m also super embarrassed going into public with it like that because she’ll say things like “Look I’m a black person!” It’s so cringy and I won’t allow it. It’s also so disrespectful and stereotypical. Today, she got upset because she wanted her hair in a “black style” and I told her to stop calling it that and I’m not doing that to her hair. She can have braids but that’s it. It angers me because my MIL lets her talk like that and act like a whole ass. She said that it doesn’t matter because she’s special needs and doesn’t understand so it doesn’t matter if I explain it to her. Personally, I’m still not having any part in this bullshittery. Thoughts?