I think you're going to be pretty cramped with 5 adults in a one bed. It, potentially, makes for some very friendly sleeping arrangements :O . There are also the potential problems of 5 adults having to share one bathroom (particularly with three girls wanting to get ready to go out). I would agree with Dean, if possible look at the lower point cost of OKW and get a 2 bedroom, that way you get 2 full bathrooms and every person/couple gets their own bed and privacy.
Would it be too embarrassing/difficult to put your sister off until a later trip? Your friends may feel a little uncomfortable if they think their participation in this trip has lead to you having to share your bed(room) with your sister. You can use the 4 people limit as a genuine excuse if you feel it's you best option out with her. I genuinely think you'll all have a much better experience if you have this trip with your friends and have a later trip with your sister so all parties can have some space and privacy.
Was your sister invited to join you? did you invite her, she decline (so you invited the other couple) ? or did she decide she would join you? IMHO if it's the latter ( and particularly as you're new to
DVC) it wouldn't be a bad thing for you to lay out the ground rules pretty early in the process or you may find she likes to invite herself (and family) along on all your vacations. It may not be the case with her, but it is the case with many relatives that they see your outlay of capital as an opportunity for them to have free holidays. Putting them "straight " inevitably happens, but the experience of those on the boards leads me to believe it is far better ( and less painful) to deal with this potential situation early and politely ( while one still can be that LOL) then to leave the situation to fester and end up losing one's temper ( and the plot) and giving it to them with both barrels.
As Cruella points out it's a touchy subject here on the boards. I can understand why people with three relatively young kids would want to break the rules and slide one kid into an airbed occassionally. I do think that 5 adults is a different situation, full of potential strife that's going to spoil your vacation. I would try to avoid that strife at all costs. As a guy, I can guarantee you your husband will be ( and if he isn't you may want to ask yourself some questions) pretty upset that any chance of him being intimate with you while in the relaxed atmosphere of a vacaton goes out the door as your sister comes in it.