I'm attracted to my 15 year old step sister

Ok first off she is not my ste sister, she's my dads girlfriends daughter. They've been together for a while. I first became attracted to her when I noticed she was gorgeous and unlike any 15 year old I've ever seen. Now it's at the point where I can not stop thinking of her in sexual / emotional ways. I feel like I will never be able act on my feelings because I am 21 and it would be irresponsible and illegal, nonetheless the feelings are still there and stronger then ever.

When were together I feel a strong connection between us both physical and mental, we both are very nervous around each other if left alone. With others, however, we are fine and can talk and laugh freely. I always see her admiring My body if we go swimming and Im sure she's seen me look at hers unless she is completely oblivious. We rarely touch; when we do I get a rush of sensation like I've never felt, i'm arroused, excited, nervous and curious and more all at once. I'm not A virgin, I know what it's like to be touched by a woman but it doesn't compare to the most innocent graze of our legs or accidental bumping. I honestly can tell she feels somethig for me but I don't k ow what it is, we are both to scared to say anything so we go on with our lives looking for any excuse to be with one another, making it seem like that's not the reason were doing it. Should I wait until she is 18 and then confess my true feelings? It feels like an eternity but Im in love and everything in my life fades to the background when I see her face. I'm so lost. Not to mention it would probably freak out our parents