How to go for a girl I've been banging's friend?

I need some advice and insight about going for this girl I like, let's call her Sabrina. As it turns out, I've had sex with her friend a couple times, we'll call her Christy. 

Long story short, I met Chrsity one night, banged her. Saw her again a couple months later and she was with her friend, Sabrina. That night, christy left the bar and went home because she was feeling sick and so I pretty much hung out with Sabrina all night. We were flirting and getting close but of course, she brought up her friend. She said, you must be pissed that Christy went home. I was like, "I'm fine, you're actually better company." She started to pull away when she noticed that I seemed to prefer her over Christy. Nothing happened that night. 

We've texted a bit since. Last weekend I bumped into her at a bar that Christy happened to be at. Sabrina didn't seem so receptive so I decided to take home Christy. As Christy and I were leaving the bar, Sabrina came up and asked her friend if she was going home with me, Christy said Yes, then Sabrina looked at me and there was this vibe goign on like Sabrina and I had something but everything entagled. 

The next day I decieded to text Sabrina. I told her, "Hey, I'd like to talk to you. What are you up to tn?" We texted a bit and go some rapport building. Then it sounded like she was down to chill that night but she may have just been playing a long. Because as I texted her if she wanted to watch the movie at my place or her's she ignored. Fast forward one week later to tonight. I see her at a club and say hi, she is extremely receptive, grabs my hand and we start venturing through the club together. We're talking and getting very close mouth to mouth wise, are lips almost touching. It is at this point that I wish I had of grabbed her head and physically made out with her. But I don't and screen to much to ensure her compliance, gay I know. And as the moment extends, she say's, "Wait, are you and Christy hooking up?" I'm caught off guard and not sure what to say. I would totally jump ship because I happened to be more drawn to Sabrina even though some guys would consider Christy hotter. But I tell her with long puauses, "We've only hooked up twice, I have no idea what it is." Then I start to get closer and I see Christy pull away and I tell her let's grab a drink. She says as friends, and I tell her what else would it be. I'm half inclined this was a test and she's interested but she may just not want to make moves on a guy her friend has hooked up with, I don't know. 

Later that night I tell her to come outside the club. She brings her friend. We chill but I can tell her attention is all over the place. As we get closer she brings up Christy again. She say's, "How's Christy?" I say, "Amazing" with a exaggerated smile and hint of facetiousness. I pick her up and try carrying her out, I tell her we're getting food. She resistes but I keep pulling her. I think half of her wants it but half of her feels bad and also wants to stay with her friend. I can tell she's not coming home on this night so I release and she runs back to her friends.

Jesus! Need some better way to handle business when she brings up Christy. Like Sabrina is cool, I'd be down to date her. I can't say the same about Christy.