Did your future brother-in-law do something silly and harmless, or was it inappropriate enough that you have concerns for your sister’s well-being? If his behavior reveals a pattern of disrespect for himself, for your sister, and for the marriage vows he is about to take, there is more at play than one evening’s indiscretion.
A good approach might be to speak with him first. Tell him that you are concerned about what you saw because of what it might mean for the future. Suggest that he talk about it with your sister — if something serious happened, it’s better for her to hear it directly from him. But if he’s defensive about it, then you probably should speak with her. If her fiancé’s behavior at his bachelor party is part of a pattern, she probably already knows. Your support and encouragement might help her to face up to it and ask herself some tough questions. This incident might prompt your sister and her fiancé to work on any underlying issues so that they are truly prepared to begin their married life together with fidelity, honesty, and respect.
from Neela Kale and the Busted Halo Question Box