family bad situations

my neices little sister was drinking and told me how her and her sisters boyfriend(they just bought a house) were kind of hitting on each other when they were drunk. she was laying in his lap and blah blah but their friends who were there and the boyfriend begged the little sister not to say anything shes 18 of course. but i feel like i need to tell her but again i dont want to cause any drama they are about to move in to their new house.nothing happened they were both drunk and he apologized. nothing vulgar but just flirting. still should i say something to my neice or just leave it alone. im worrieed that if it comes out the fact that i knew it just puts me in a bad situation. nothing happened there was flirting saying things that werent vulgar but still shouldnt have been said. both were drunk but her best friend little sister boyfriend all know and none of them want to tell her becausse they are about to move in to their new house. what should i do