YTA. Your efforts to hold your son responsible sucked, not only that YOU are partly responsible for giving them sex education. How is he in his early teens and still not know what a period is? He doesn't lack empathy but he needs to work on giving it, especially if he didn't actually apologize. I think you should have taken action immediately when he called her period disgusting though and not waited a few hours after it was OBVIOUS she'd been crying, perhaps immediately correcting your son in front of your daughter would've help her understand that he really had no idea and then that's when he should apologize, that would have perhaps cleared things up and prevented your daughter from any circulating negative thoughts about her body which will definitely affect her in the long run during those hours. I don't understand waiting out your children being upset, she needed it from the beginning and you weren't there for her. Try to work on your son on giving empathy, however, your daughter needs empathy right now.