The part where...

I'm just as lost about it myself.

From reading some of the other posts on here I gathered that Christina had suppressed those memories and so when she left them she still believed in her mind that her father didn't do anything to Kelly. She knew it in the back of her mind because when she is talking to Kelly, Kelly says to her you know and Christina is still fighting it not believing it. Although I am a tad confused because just before that her husband says if there is something she knows then she should be there for her sister and Christina gets upset as if he is assuming something happened to her and he was right. They truly didnt write that well if she knew but just did not want to admit it yet. It was not until her little brother screamed that she totally realized it was true and that was when she rushed back to her house.

Christina was probably in denial that her father would abuse the other children. she probably believed that he abused her personally, because she was his "favourite", and therefore that other children wouldn't be in danger around him. it wasn't until Patrick exhibited signs of being abused that she realized her father was abusing them all and that she had left her own daughter and her sister in danger. i think, of all the characters in this movie, i felt the worst for Christina.