21 year old interested in 16 year old girl

So I have two sisters who are twins, over the last year or so I've noticed a guy who has been flirty on social media with my sisters. I already know he's not shy with girls their age because he has been in an intimate relationship with their friend when she was 15. I dont know if I should do anything or leave it, I know it's technically illegal but I don't want to ruin some guys life if nothing has happened with my sisters. They're not mature enough to be having those sort of relationships with guys his age, obviously they've not developed fully mentally or physically. I just feel like it's really weird and I don't know if I'm in the right to talk to him or leave it. I don't like the idea of someone whose older and more experienced to take advantage of two adolescents. Plus my mum is an alcoholic and doesn't care for my sisters at all, I've not spoken to my mum for two years because he addiction made my childhood painful, they have a different dad to me and he hasn't seen my sisters for five years