My sister is 18yr and has a intellectual disability decided to leave home to live with an 40yr old man

Location: NYC

This man has been in contact with my sister when she was a minor and I did whatever I could to be able to stop this communication with them. My mom didn’t do much because we did not know how far this would go. After few months after her 18th birthday we had found out that instead of hanging with her friend she has been secretly going out with this man. Once she realized we had known she was “afraid” to come back home, leaving her belongings behind. We have tried to go to our local precinct which the first cop seems to not care and another cop step in whom we told our situation and told us that they can’t do nothing. I called to make a wellness check on her but police could not get inside since the apt has no doorbell. My sister is unable to get a high school diploma as she is an IEP student. What can I do?