What should i do about my 10 year old sister?

She's bossing my mom around. Not only that but my little cousin jaydon spent the night last night and kept me up all night. Then me and my boyfriend heard her and my cousin jaydon messing with her cat, only it wasn't the fun kind of messing, we heard my cousin say " look maria, scooter has my ***** in his mouth" then just heard them laughing. Then my cousin said "well he must have liked it cuz he's licking my toe" then they started laughing again. I'm afraid that my sister might keep going on with what happened last night. I tried telling my mom, and my mom talked to my little sister but she said nothing happened. This is my little sisters behavior ALL THE TIME. She does something to me or she uses my fears against me then turns around and acts all innocent to my mom. I'm getting sick and tired of this bullshit that she does. And I'm also 8 months pregnant and I can only handle so much. She also told me to my face that "she wished my baby would die right when its born" and that is something you don't tell anybody NO MATTER HOW MAD YOU ARE. I need some help and advice becuz I can no longer deal with this and I hate seeing my mom getting pushed around by this brat. What shall I do?