What would you do if your mother..?

ok to start off me and my mom never had a good relationship. She divorced my Dad because he was an alcoholic we move away from him and she starts drinking and sleeping with a diff guy everyday. She ends up meeting this guy and marries him, they was married for 7 years so this was a guy that we all trusted. one day my 6 year old daughter comes to me telling me that papaw touched her. I freaked out! I took her to ER they called cops and CPS. She told CPS her story and about 3 months later he was arrested. He is now in prison! Anyways my mom gets with his cousin. Since that happened to my lil girl I am all the time on sex offenders because apparently you never know these days and HER BF was a registered sex offender. She told me well it was his 16 year old daughter that lied on him to try to get her way. Well i went to the prosecutes office because he knew what my daughter just went threw. I told him the story she told me and he said that is not the case at all he said DO NOT let no kids around him that him and his dads case was one of the worst cases in our county. So i told my mom and and she did not believe me so we have not seen and talked for a very long time. ok my question is my sister has 3 kids from 3 months to 7years old and she is into the weekend bar thing and has my mom and her bf watch those kids knowing that he is a child molester. What do you think I should do? I am stuck she wont Liston to me at all atc. please help