What should i do about my boyfriend’s sister?

We live with my boyfriends parents, he has A LOT of siblings, his parents do foster care/adoption. One of his younger sisters is almost 9 years old. She has a very smart mouth, and acts a lot older than she is. She got mad at me and my boyfriend and said she wished she would kill us. Then she went in the laundry room and took my daughters clothes (my daughter is almost 6 months old) and was hitting them everywhere and saying “heck the baby, she belongs in the dumpster”. Then she starts calling me fat and saying “that’s why you’re grandpa died” . My grandpa died a month before my daughter was born and it took a really big toll on me. I lived with him and my grandma for a very long time and took care of him when she couldn’t. and about her calling me fat, i’ve struggled with my body image A LOT since giving birth. it broke my heart to hear her say these things, and it made me more angry than i’ve ever been! Obviously i didn’t blow up on her because she’s a kid, but i’m still extremely upset. i cried for a really long time. I’m 20 years old, so things like this still upset me (even tho i feel like they would upset anyone!!!!) I don’t really know what to do, or how to feel. If anything, i just wanted a place to rant.