I have been to roomate hell and back - the drug manufacturer, the mama's baby (mommy, I have a hangnail! - she lived less than a mile from school in a semi-mansion, and was intent on having her own room), the nicaraguan princess (very rich, very spoiled, talked about me constantly en Espanol).
If she wants to move out - let her go. I know its a disturbing and upsetting situation. Every time I had to move out, I was very upset for awhile. But you could possibly end up with a private room, which would be a really good thing. I'm also an only child and have my "ways" and its just easier to have your own room.
Some people were meant to live in a 10X5 box with others and some weren't.
It can work out okay though, I found my current roomates (we share an apartment) kind of randomly - their RA told me they had empty rooms. And I've become great friends with 2 of them - I'm going to even be a bridesmaid in one of their weddings.
Just hang in there, sweetie, and remember that your sisters think you're cool, we think you're great, and many others do too.
