I dont know who to believe, husband or sister? help!?

Ok so here's the thing so my sister is 18 in college and she came down one sat night to hang out and drink. She came with her boyfriend. Well my sister claims that while my husband was making her a drink he asked her to show him her ! Well she was WAY drunk we all know, my husband was not. She's not his type at all, she's pretty over weight and has bad acne. Not that that matters, but he doesn't think twice about her. So what I remember from the time she claims it happened (iwas in the living room) was they went in I told him to only give her pepsi and she was begging him for vodka (cuz she was already drunk I wanted her to stop) but they went in and they started whispering I said "hey there's no whipsering say it loud and proud" thinking it was them taliking about giving her vodka. She laughed really hard and said were not talking. Then her bf went into the kitchen and she said something about how he couldn't come in yet and then something about he couldnt see it (talking to bf) I thought they were either talking about something on a phone or the drink being made. She came out of the kitchen fine and didn't say anything. But I guess a few hrs later she told her bf something abut my husbad trying to take advantage of her and asking her 4 times to show him her s. Its not like my husband at all! He is 26 and is very committed to me and I know he would never cheat or do anything jeopardize us not just because he loves me but also because we have 2 little girls and he feels just as I do they don't deserve there parents being unfaithful. Anyways she all of a sudden after we thought she was asleep in bed came dwon the hall and said he asked to see my 4 f*** times. I seriously thought she was joking so did everyone else and we were laughing. Till she got mad at me and went to the bathroom and told me not to talk to her. Well I followed her and she was crying, I asked her what was wrong and she said I wouldn't believe her and she told me again.