How do I handle this situation?

My sister is 29 and she had a daughter who is a few months, and the dad don't help her with her at all and I think she is still dealing with him. And she also had a 5 year old son who's dad is in prison, she had been leaving him at my house for months without even coming to check on him or bring him changing clothes, and he is very bad he doesn't listen to anyone, he runs outs side alone and ever since he had been here he stress me out a lot.. I stay with my mom and my mom won't tell my sister to take him home.