I'm not a guy nor am I a lesbian. But I would honestly be mad/pissed that someone I was seeing spent more time with my younger sibling than with myself when we were on vacation. I mean my goodness the vacation is supposed to be fun for everyone and sure the significant other can chat and play cards or something like that for fun with the younger sibling. But if my significant other is getting drunk and throwing up with my younger sibling. I would be very mad. I mean they would be the adult in the situation and engaging in under age drinking with my sibling is not cool. Nor would it be ok for the significant other think it is much more fun to go drinking with someone else while we are on vacation instead of having fun with and everyone else in ways that have nothing to do with getting THAT drunk. Just not cool and would be a very immature side of my significant other that I would never respect.