My parents are divorced. My sister and I live with my dad and we see my mom every other week. My mom lives with an felon ex-convict with a semi-violent past, it is legal for him to be in the house when we visit? We are afraid to be around him. He only got out of prison in May and moved in with mom in July.

I doubt that this man's mere presence in your mother's house is illegal, although I can certainly understand why you are somewhat concerned.  Unless he remains subject to restrictions associated with his parole, he is probably free to associate with whomever he wishes - and so is your mother.  Much depends, of course, on his actual crime of conviction (you didn't really explain what you meant by "semi-violent"), how old it is, and what positive steps he has taken since then to improve himself.  Your own age (as well as your sister's) might make a difference, too.

Your parents could also review their own divorce decree to determine if the family court imposed any special rules about live-in boyfriends or girlfriends.  Your father might have the option of asking the judge to re-examine the case in light of the current situation at your mother's house, but he would probably have to do a lot more than simply say, "She's living with a bad person."  He would have to show that this man's history and current behavior constitute a tangible risk to your safety.

You should also remember that divorcing parents generally have the right to make a fresh start, and find new partners who make them happy, without having to worry about ex-spouses meddling in their personal lives.  However, if domestic violence were to occur in either house (even if you're not present at the time), then that would be a reason to take immediate action.