Boyfriend break up wit me :((?

My sister has a boyfriend like this, they have been together for 3 years and they have a 6 month old baby together, he is super possessive and say if my sister come with me to walmart for some groceries he will say something like "oh so are you going to go do some guy" now my sister and him have broken up time and time again and he says he will stop but never does. I say stay broken up, trust me if you get together again it will get worse and worse over time and now matter how much he says hes sorry he will do it again. I also had an ex like this and it would have saved me a lot of time if someone told me what im telling you.