IMO, living with family or friends, is fine and dandy for a little while, but then just causes unnecessary drama and stress. With that being said, buying your own house is also stressful. You have a monthly mortgage payment, and if anything goes wrong with the house, you have to pay for the damages out of pocket. So in this situation, you need to chose the lesser of two evils. Good luck.
Just depends on your financial stability and how much privacy you want! If you can afford it, getting your own place would be a lot better because rent money is just wasted money going down the drain... Unless your sister is letting you live there for free (;