I gotta problem with my best friends girl?!?

Well not really a problem more like a dilemma. Me and Friend eric have been friend for years now ever since freshman year in high school. but when i met him he had this Girlfriend named Alisha that he had been with for about 2 years. Alisha was younger than him by a couple years and everything was cool at first and she was always cool with me. We grew more as friends she became more like a sister over time and i just left it there and never went any further because of the loyalty to Eric but as time went on she grew apart from him and eventually dumped him. But there was a problem it wasn't so easy just to stop talking to her just because he stopped being with her. At that time me and her got closer not in a ual way or intimate way we just had a lot of respect for each other and i never wanted to complicate that with or any other kind of relationship. Its not that she isn't a really pretty girl its just that she wasn't mines to begin with. I just never saw her as that, mainly because i was older than her and she was a lot younger when i met her.But recently we started spending alot more time with her. She even spent the night over my house this past weekend but now i have another problem because she has a friend Tracy thats really attractive and she likes me alot her too and but every time me and her get closer i can see out the corner of my eye Alisha getting real upset and then ill see her give Tracy a look and Tracy looks back like what? I just dont know what to do. Because its not like i have mutual feelings i just cant break that trust me and Eric have. Can somebody give me some actual advice?