the characters i feel sorriest for, in order
1. Christina - the easy answer: she has been suffering the longest. even though she had repressed her memory or was in massive amounts of denial, that is still suffering. she also has the residual guilt of leaving her siblings to a similar fate. even though it certainly wasn't her fault or her responsibility, she's going to feel it was, as evidenced by her crying and apologizing to her siblings when she realizes they've been abused.
2. Patrick - he has been being abused for the shortest period of time, but he has Kelly as an added tormentor - locking him in the bathroom, the scariest place in the world for him. and when he reaches out to her to tell her he believes her, making his first attempt to open up in the same way she had, she rudely brushes him off.
3. Kelly - for obvious reasons, and for shouldering the blame her mother heaped on her. man i hated that mother, almost as much as i hated the father. maybe even more.
4. Nini - she doesn't come across as a sympathetic character, but give it a bit of thought beyond the scope of the movie. she hasn't been abused, so of course she believes her dad over her bully of a sister. there will always be doubt in her mind, instead of a clear knowledge of the kind of monster the father actually is. that, in itself, is hard. if she continues to have a relationship with him, she will be ostracized from her siblings, but if she separates herself from him, we've already seen the manipulation he is capable of and the level of guilt he induces in his children. she will be torn between the two sides. the only way she won't be torn is if, once he gets out of jail in 10 months and moves back in with the mother, who will likely be waiting with open arms, he begins abusing Nini and she knows for certain what he is like. and that's an even sadder, scarier option.
5. Christina's husband - survivors of sexual abuse, especially when it involves childhood incest, have a lot of emotional garbage they shoulder. guilt, feeling dirty, feeling like they brought it on themselves, not wanting to be touched in a sexual way, the list goes on. Christina doesn't appear to have worked through her experiences yet. she is going to need her husband's support as she works through it now - and her husband is going to hear some harrowing stuff about the woman he loves as she does. he will experience a feeling of helplessness, wanting to fix things for her but not being able to, which is a difficult thing for a man to go through.
6. to a certain degree, the father - most abusers are not first-generation abusers. there is a strong likelihood that the father was, himself, abused as a child. now i'm a firm believer that it doesn't matter what leads you to do something - you are ultimately responsible for having done it - so i'm not letting him off the hook for the abuse he performed. but i can certainly sympathize with the abuse he likely endured, and then he has the added guilt of knowing he perpetuated it and damaged his own children in the same way he had been damaged.
that's it. i'll point out that i haven't accidentally missed the mother. i found her character the most repugnant of all.