This past academic year I roomed with my two best friends from high school. By October Friend A and I were no longer speaking to Friend B. Friend B was never there, always had her area a royal mess AND showered once every couple of days. Friend A and I approached Friend B about the situation in a mature manner and she went ballistic saying that she just didn't have time to shower and that she can't believe we didn't understand. In February of this year Friend B and I started talking again, mostly because she moved out of the dorm we were sharing in December. We've been friends since 7th grade, so it was just a matter of time before we began talking again. Oh yeah, and she does shower on a regular basis now.
Because of how Friend B reacted, I would suggest that you not mention hygiene issues to your sister. I know that not everybody would react like that, but because Friend B and I did not talk for months because of it, I think it would be best to not mention hygiene to your sister. I say that you tell your sister that you think living together may be a problem, seeing as how you are a person who likes your living area a bit tidier than she does. You may also want to mention something about the BF moving in also, something about if you were to get another room, there would be more room for them both in the original suite.
You are in a difficult situation and I wish you the best of luck!
Off subject side not: Friend A and I stopped talking in December. It was so hard spring semester because not only were we living together and not talking, she is also one of my sisters. Living with friends can be difficult and I definitely do not recommend it. Some people can do it, but a good number can't.
