Audio problem: During the girls' follow-up discussion after Tracy has sex with Ravi, Evie laughs and says, "What? We didn't go over that one." However, her lips aren't moving when she speaks the second sentence; the second part was obviously dubbed in later.
Continuity mistake: When Tracy first goes to meet Evie, she gets on the bus with no purse. She even leaves her house without one. When she sees Evie in the store, she has a purse on her shoulder.
Answer: Tracy acted like she had done everything Evie did before. She pretended she was already bad to become friends with them, so Evie thought she had found someone who was like her. Evie was definitely also in love with Tracy, but was also jealous since Tracy had a family who loved her. She wanted that. Yes she used Tracy, but she also cared about her.
I don't think Evie was romantically in love with Tracy. It was more about her wanting to *be* Tracy, with a mother like Tracy's. Also, she seemed to know that Tracy was only pretending to be "bad." I think Tracy's innocence made Evie sad, because there was a time when Evie was more like her.