Rude SIL ☹️

So my SIL is 7 weeks further along than i am. You’d think she was 30 weeks the way she acts but she’s only 15-16. So a majority of this drama has started because her & my brother decided they wanted to be there when i told my dad about me being pregnant (mind you i never once asked/implied i wanted them there, i only said okay to be nice). Their excuse was “we were nervous telling them too”. But she kept saying over & over “i can’t wait to see how he reacts” & i really think it was just her being nosy & wanted to see if he was more excited for mine than theirs (he wasn’t, he was more worried than anything really). Soo anyways we got there before them & went ahead & told him because we couldn’t be there long as we had something to do after. Well she was pissy the whole night after, we left earlier than them & she waited until we left to talk crap about us & say how she “didn’t want anything to be a competition between us”. Well guess who’s the one making it a competition??? Not i. I had to start staying with my mom bc of issues at my boyfriends house & apparently she’s pissy about that too??? They came over the other day & my stepdad joked about how i could help pick up something heavy bc i “wasn’t that far pregnant (7 weeks at the time)”. She laughed her ass off saying “he says shes not even that far pregnant.” Then the whole night she would talk at me & not even acknowledged when i would respond to whatever she’d say. She’s just so rude & condescending to the point i don’t even want to be around her or my brother at this point. I don’t want it to be like that bc i want to be in my niece/nephews life but i cannot deal with her. How do i even try to fix this when this is just a rude person who doesn’t care one bit about anyone else’s feelings?? Idk what my brother tells his women but his last one & this one have hated me for 0 reason & started drama with our whole family for no reason. Like my brother is just so mistreated when he’s definitely not. I would be the first one to stick up for him if he was. Also just to add my brother is 4 years older than me & she’s 6-7 years older than me & literally acts like a mean girl in highschool. I don’t know what to do 😢☹️ i love my brother to death but i don’t like her at all. Also please note a majority of the family doesn’t like her but act nice to keep the peace. They’ve been together just over a year, got married after 6 months & didn’t even know each other but maybe a few weeks before they got together so not like they’ve been lifelong friends. She also broke up with her longtime bf of several years to date my brother. What is up with this girl???? I’ve never been anything but nice to her, even when she’s rude towards me.