Should i confront my sister in law or just call social services?

my husband and i live with my parent in laws as well as his brother and girlfriend. she moved in 2 years ago with her daughters 2 small daughter who are 4 and 5 right now. ive never liked the way she desciplines her daughters or the fact that she lets my brotherinlaw discipline them too. when theyre in trouble i think they get hit becuase i hear sounds like skin contact and then they put them in timeout in theyre hallway which is closed off to the rest of us with theyre hands up in the air. ive asked the girls before what happens to them and once they told me theyre mom hit theyre hands with a shoe and pinches them and a whole bunch of other stuff althought i checked them and couldnt find anythong except red marks. my mother in law says shes told her stuff and has proof but i know it isnt true and ive always been scared to say something cuz i dont know whos side she would take and how the family would react. today my niece came home and told me her mom scratched her on her beck and punched her on her forehead ans showed me proof. i took pictures and videotaped her telling me excactly what happened. i dont know what to do all i know is that im sick and tired of allthis. how can i let this happen in my own house. i feel horrible but dont know what to do. if i call social services will they take them away when my sister in law doesnt have family here. would my mother in law be able to keep them even though theyre not related. im torn and need to make a decsion. or should i confront my sister in law n tell her if she does it again im calling social services.