Kira is a porn actress. Her career is based upon appearing to love sex in every form. I think the only one in the family she's truly interested in is her sister, Ann. She may not have planned on using Max to get to her sister, but she will definitely work with what she is given. Maybe she enjoyed fucking him, but on the other hand, she was definitely motivated to make him think she did. The entire time, she's been working towards her goal of seducing her sister, while foisting Max off on Lisa for the most part.
Now that she has Ann, it's on to Kira's next goal. She doesn't want to be a porn actress anymore. She wants to be behind the camera, and if that involved screwing Max on camera to make that happen, so be it. Now she's moving the porn focus to Max, Ann, Lisa, and Alice, while she becomes a successful director and producer of incest porn movies.
Or maybe I am reading too much into it. I'd like to believe that Kira is the subtle, devious mastermind that Eric should have been. Except the best part is, she's a girl, and you get to fuck her a couple of times. I think it matches the facts fairly well, but that could just be a coincidence. But Kira doesn't really seem interested in having sex with Max except when it suits her purposes, and she's managed to somehow get things to work out just the way she wants them. She'll use her body to get whatever she needs, whether it be with Max or Eric. But for Max at least I think there is general affection. Once she has what she needs, she doesn't really want to sleep with him again though, unless there's something else she needs.