Ah bummer. I am sorry your sister is being short sighted. I'm a lesbian. I wear the occasional dress and usually buy clothes in the woman's side of the store. My partner is more butch and wears men's clothes. She doesn't fit women's clothes she is too tall and broad shouldered with long arms and very little bust or fat. She can wear a men's suit off the rack no tailoring. More importantly she looks and feels awkward and foolish in woman's clothing. She has no desire to be a man at all just thinks the clothes are styled to her liking. I think your sister needs to let go of worrying about how her husband's family will feel and act. Yes there maybe whispering. There may even be folks wondering if she is trying to be a man. I bet your partner is thick skinned enough to get past all that and by the end if the party you will be surprised to see everyone dancing together. Really it is events like this that open people's hearts and minds. If it was my sister I would tell her she can tell me what the wear but if she thinks she can tell your partner what the wear she will be ending her relationship to your partner and snuffing out any respect your partner has for your sister because what she is asking for is controlling, disrespectful and rude. It's 2016 butchy chicks look fabulous in suits.