Going out with someone you already live with is a bit weird and carries inherent problems with it. What if it doesn't work out? What if it all goes sour?
Notionally, however, you haven't grown up with her, you don't have an established long-term sibling relationship, nor are you biologically related. It wouldn't be any different from taking on a mother and daughter as tenants and falling for the daughter. Honestly, I think it's quite understandable if you're anything like you're father and she's anything like her mother... if they found each other attractive, it's not too surprising that you might also. I think things are early enough on that, if you have the will to make it work, you'd probably have a shot at it.
The only thing I'd strongly suggest (even though it might seem like the hardest thing in the world) is to talk to your parents about it. Yes, they might freak out about it, but better for them to freak out before anything has happened rather than after, and trust me, parents have a way of knowing when things are happening and finding out about stuff- they'll find out one way or another.