Complicated Situation Please Help

So I've been very close friends with this girl for a while. Let's call her Mary. We dated a couple years ago (we're both seniors in high school right now) we broke up because we became interested in other people. She was in a friend who she was interested in before me, and I was very interested in her sister. We'll call her sister Emma. I spent the better part of a year chasing her sister, and we actually became very close, going to Maroon 5 concerts, playing basketball, eating fast food and living up the good weather. She met someone else and they've been dating for the past year and a few months. He's sort of a prick. Anyways, Mary broke up with her boyfriend, partly (mostly) because she started having feelings for me again, and he was becoming alien and foreign to her. We're in this sort of friends with benefits thing right now. And I could see myself with her, but I couldn't really see anything passionate happening. I think at best I would feel comfortable. If something's bothering me, and I try telling her, she becomes cold and makes it all about her. But she's really sweet. She's told me how she's madly in love with me, and how she's terrified of losing me, especially to her sister, who I'm still pretty interested in. Emma and her boyfriend are in a rough patch right now, and although they have t broken up yet, it's given me the chance to reignite our friendship and get on her good side. Mary doesn't know I'm into her sister again, what do I do? I can't hurt her, but I also can't control who I have feelings for. I'm really torn here and could really use some help. Thanks.