23 year-olds dating or trying to date 15-year olds deserve to have the **** kicked out of them, so based on that information I can definitely empathise. That said, you should have at least tried to ascertain who this guy was and what exactly his relationship was with your sister before you went all medieval on his ass. You got lucky in that he's a cokehead scumbag (if that's what he is), but your sister might just as well have told him she's 19, and/or he might have been the most decent guy on the planet, and/or there may have been nothing going on at all except that he held her hand at that moment. All in all a bit rash.