Why am I getting involved in something that I don’t care about !!!?

Okay so last year 2017 she started dating this guy it was about 5 months after she told me because she said she was AfraidI wouldn’t approve but I told her I did.Shes 17 and he’s “24”.he’s my moms friend brother. And his family really dislike her they talk shit about her saying she’s a hoe and etc.... and that really bothered me .his sister keeps saying he’s only using her and this and that and he don’t like her being friend w me to the point where he told her to stop hanging out w me and she said no so lately she’s been really distant from me and we don’t even. Text anymore not hang out anymore .and yesterday my mom was telling me to stay away from her because they are talking shit and she does not want me involved and the sister says she’s a little slut for going out to party’s and how she goes to his room and have sex and how after she leaves he brin out the sheets because he feel disgusted and my mom really loved her but lately my mom now dislikes her and my mom friend is also saying I’m skipping to go fuck w other guys and it really bothers me to now find out he’s really 27 and they are dragging me in this. And I’m not even let if the relationship !!! And that how he says I’m a bad infulenced and that she’s telling my secrets to him and he is telling his sister and that could really get me in trouble . What do I do!??!?! I tried asking her but she says she don’t tell my business to him