My guess is that she feels comfortable and safe around you. She probably thinks of you more as a brother she gets along with.
I would not try to find out if her feelings mean more than that if you like your living situation. If she doesn't have feelings for you she will feel embarrassed and things will get ackward. If she does have feeling for you it will be very much the same AND if she does and you two decide to do something about it what happens when one of you isn't interested anymore? You have to move.
Think of yourself as sort of Will & Grace roommates. Very close, even affectionate, but not lovers. If the time ever comes when you can risk moving out, and her behavior continues - it could be worth checking out, but not until then.
She could also just be a tease!
Stay cool and detached and act like her cousin.
Susan