"I'm your brother?"

Diana feel like something isn't right about the girl. Firstly; she was so direct with her intentions.

Secondly; she's carefree like she got nothing to hide.

If she's really the killer; it could only mean one of these 2. 

It's either she knew about Diana's intention and was sure she will do her bidding anyway; which is why she didn't bother stressing herself to come up with a plan to get her since Diana is also trying to get her.

She's just a girl who's into females and was really interested in her.

Diana started calculating all odds, but doubt the second one and hope the first one to not be true.

If it's the first, then she's really in danger and didn't have a choice but to get her neck deeper in it. Even if the girl knew; she still gotta play along.

If it's the other ones; then she will be starring up the girl's feelings and break her heart when she discovered she's clean and left her.