My sister needs help. Advice?

Little back story, my sister met guy at work. Was all good until like 4 years ago. He threw a fit cause my mom invited my sisters old friends down to see her. She was really close him. He shut her phone off, smashed her car, and did other horrible things. My brother and I ended up staying the night at her house at the time for her safety. My sister has right now 3 kids. 2 of them with different dad(twins) and other with him..(ugh horrible horrible choice and idea) He would also sometimes hit her. Now for the past two years he says these really horrible things to her. Like “she a horrible mother”, “she can’t do shit”, and “he has raised the kids”. He barely work, my sister takes care of her kids, and on his days off he prefers go fish instead of take care of his son. When he comes home he doesn’t even bother helping out. Just sometimes cook. He always been bad to my sister. We all have told her leave and she can come back and live with us until she gets back on her feet but just won’t. Will.. now it is starting to affect her other son(twin they are only 11. One boy and one girl). He has been back talking a lot, gets angry really fast and starts trying fight with you, always “YOUR NOT MY BOSS!” “You don’t tell me what to do”, always just rebellious stuff, he sometimes hits somebody or his sister a lot, he has gotten in few fights in 5th grade, has stole 100 dollars(my dad took care that and took him to jail and cop had a talk with him. We had hoped it would’ve stop there but it didn’t..) and one day he came to our house and hit one of our dogs really hard. My parents got really mad at everybody and just kicked them all out. Cause my mom couldnt handle it no more. She knows all the bad behavior coming from my sister bf or “fiancé” whatever she wants call him because these are all the same behavior he does. That I have seen and heard of.

Ps. A long time ago, like three years ago. My grandma (we no longer had or will contact again) reasons why we contact her anymore cause she got me involved in it when I should’ve never been inlvoled in it. I quickly told my parents when they came home. I should’ve never been involved. the kids said they get hit.I know sometimes my sister bf would watch them. I just now wonder if he has hit the kids. We just thought it was made up cause sounded so made up cause my sister would’ve never hit her kids. I haven’t heard anything about the kids getting hit or seeing anything on them. I know my niece would’ve told me. We are supper close.

Also i know my sister sometimes has sleep in the baby bedroom on the floor cause her bf kicks her out the room for awhile..

Idk what to do anymore.. any advice!? We have tried talking to both of them(my sister and the kids) but.. ugh just nothing sometimes :/