I guess you missed the part that eric is blackmailing Kira for sex to save Max.
That is if you go to Kira at 3 am several times and get the BJ.
When the Sacrifice opportunity came up, I had to make a decision. In my mind, Kira was choosing to concede to Eric's demands for the family. She is a more worldly adult and had chosen this path. She was letting Eric screw her with her eyes wide open. She knows that Max is not happy with this event. If, on the other hand, Max would choose to offer up his older sister (because he's already having sexual interactions with Kira?), that would have led to Max's corruption.
In my first run, I cooperated with Eric for a time in order to gain his trust and get the money out of him to help achieve his downfall, then once I felt I was ready (I actually wasn't. I should have waited a bit more to get more cash), I changed to war and kept at it. In a way, this was sacrificing Ann for a time, but trying to undo his influence on her was nearly impossible. Instead, I made sure Max's two sisters were not corrupted by Eric while playing him.