Four years ago my sister and I purchased a house together and christened it Buttermilk Cottage. She has the downstairs and the upstairs is mine. We both share a love for decorating but have different styles, she is French Country, I'm more Shabby Chic. Having our own spaces is an ideal arrangement for us, when we get tired of something, we can simply change it. We don't have to worry about stepping on the other's toes. We can also bounce ideas off each other, get recommendations, and keep the other grounded.
What could be better than sharing a house you love, with your sister and best friend. I say not much.