Ummm help lol?

yea so me and my other boy cousins are always talking about and stuff and onec we were drunk and talking about it in fron of my 5 year old cousin, today me and my cousin went to pick him up from his friends house and we saw his sister "orally" pleasuring him. we immediatly pulled him off and told the girl never to do it again. how should we tell him not to.