Can you have your kids taken off you because you ex beat you up?

my brother moved into my house with his girlfriend and i only found out yesterday that she has had her kids taken off her she told me it was because her ex used to beat her up so the social told her it was him or the kids and so she puck the kids. but they tuck the kids off her any way wich i find very hard to believe but she has told other members of my family a different stories so i just wanted to now if it is possible for the social to take your kids off you for that reason i need to now because i have a 4 year old son and i need to now if i should kick her out?