kinda just need support tbh..

Sorry for how long this is in advance. 
So I live with my sister, her husband and their kids. I had to move in because me and my ex broke up (was messy and I didn't have much choice either here or homeless)
Since I've been here my depressions got worse, so has my anxiety and there's even talk with my mental health doctors that I may be developing worse issues because of this place. 
All my brother in law does is shout, get angry, expect me to look after the kids when my sisters out or throw hurtful comments at me. My sister doesn't stand up for me, she's clearly down trodden by him. I've stood up for myself a couple of times and ended up nearly being homeless on the streets (trust me I've actually been tempted to just do it.) and when I do my sister has a go at me like I'm in the wrong for shouting back or telling him I won't be spoken to like shit when he starts. 
I've recently stayed out for the past near 3 weeks just to get my self out of the situation and only came back as my friends couldn't let me stay for more time unfortunately. But when I came back he created me by shouting "it's back" and giving me a dirty look as if I'm just sort on his shoe or something. 
I haven't slept all night because I just don't feel safe or happy here. All he does is stare at my chest or be horrid to me. Sometimes at the same time. It's really upsetting me. (I've told my sister about him looking and she went crazy calling me a liar.) 
I can't do anything to get my own home faster but I just really need support. I'm 20, I'm not a mom but he expects me to constantly look after his kids when my sister is here, he even tries to force me into watching them by going to the bathroom 30 seconds before my sister leaves so I'm stuck with them until he HAS to interact, not wants to. Has to. 
He shouts at me for not using my life how he expects me to, like I enjoy looking through coins to see if I come across anything rare or different and he full blown screamed at me about it. It made me feel two foot tall. I just don't know what to do anymore :/ my sister won't leave the situation, I can't leave unless I wanna sleep on the street and I can't see myself leaving her and my nephews with him when all he does is shout at them too. 
I just need some help, advice or support you know?