This isn't my disabled sister, it's my twin sister. We have a 3 bedroom bi-level apartment with 2 bathrooms (one bedroom is our office), so space isn't going to be a problem and I know my sister and fiancee would NEVER even THINK about each other like that.
He comes and stays with us already so they don't have to get used to each other being around, boundaries are set and in place. Me and him have discussed this arrangement (among others) and he has no problem with it. We discussed living apart even after marriage and that was a solid plan, until we realized that we didn't really WANT to be apart much longer. Our original was to move to Germany and start having a house built there, but I own a business and need to be here for about the next 2 years. He can take 2 years off from his business (it's a family business).
If we decide to stay here longer and buy a house, then my sister is welcome to come with me.