Reread those night talks they have at start and yes in the end not acting and staying with the dad , not stopping him and keeping the status quo since she loved him is supporting the dad in that whatever her reasons, especially from the pov of the abused she chose the dad over him emotionally speaking and that not once or twice, but years and years continually.
Sad thing about those things, there is often no real blame to attach often and everyone had their reasons if look at it more logically from the outside, but lives are fucked up for rest of their lives if you are the victim and believe me or not, all those times you say you love them while letting the abuse continue, you do give mixed signals at best to them, since you are not important enough to even keep them even safe at least, whatever you say or how often you hug them.
Especially when you talk about kids who are still in and going through all their normal development stages and do not have the defenses developed needed yet, that fucks them up for life even with lots of therapy later if they are lucky.