Family affair, what can I do?

It started about 3 weeks ago. At about 9:30PM I walked into my 12 year-old sister's room to steal her DVD player for the night, seeing as she wasn't using it (she goes to bed at 9) and saw her sprawled out naked with no covers. At first I didn't think too much of it cause it's about 102 degress here in Arizona, but then I noticed her hand was on her private area. I almost laughed, but for some reason I was more aroused than I was amused. Now I didn't come here to be ridiculed for being abnormal or "sick", I came here for honest help and answers. So, my hormones getting the best of me, I decided to lay down with her. I then took her hand and used it to rub her private area. Then she smiled, and I realized she had been awake the whole time. And she kissed me. It went from there. Now, every night she comes to my room in the middle of the night to have sex, and I feel that I love her as a sexual partner, but the better half of me tells me that it is wrong. I don't want this to continue, but I don't know how to stop it without hurting her feelings. I still love her as a sister, and I'm also afraid that it will never be the same after this. I'm afraid that if it stops, we will feel awkward around eachother and I don't want that. What should I do?