Sorry, I'm on the side you probably don't want to hear but if it's a family cabin, then your sister should be able to have a happy wedding there too. I would work with her a bit to make sure that they're both individual ex, different caterers, DJs, colours. Maybe have the ceremony in a different location, one on the deck, one in the garden, etc? But there's no reason that you both can't have beautiful weddings there. I also like inloveinkorea's comment that it is just a day, you don't get a week or a month and you certainly don't want it to cost you a sister. Scream really loudly in your own home and then put on a smile and go look at dresses with your sister. Now you get to have your nails and hair done twice in one month!!