Older siblings tend to always see their little brothers and sisters as kids even when they're well into adulthood, and it's easy to forget that the adult in front of you isn't still the little kid who used to steal all your clothes and cry about the ghosts in their closet. Sometimes younger siblings will overcompensate when trying to prove their maturity to the family and stoke the old flames of competition from childhood sibling rivalry.
It's one thing to have an innocent and quiet longtime crush on someone's spouse, but openly flirting with them at a family gathering and challenging your sister to a "may the best woman win the man who is already your husband" duel is a great way to ruin the mood. Respecting boundaries and protecting each other's happiness is all part of being in a family, so putting the moves on your sister's partner of seven years might not be the best idea for a birthday dinner. Save the sisterly drama for the Kardashians, kids!