So My Sisters Boyfriend .... Dave (13) Sister (12) R

so my sisters boyfriend .... Dave (13) sister (12) r getting real close and there kissing and everything and he's supposaly getting really touchy and I spoke with his ex Gf and she said he had done other stuff way worse before ... idk if he has some kind of desese or something to want to do this kind of stuff ... I'm just really worried and don't know what to do