Hi Ami - I hope you don't mind me commenting on your very vulnerable post, someone that you don't know at all.
I think it is perfectly understandable to feel betrayed and angry when someone you love does not stand up for you in the face of another who has hurt you -- when that person actually wants you to pretend and lie so that they won't have to feel uncomfortable or vulnerable.
To suspend your own feelings and to think of whether or not your father might be going through a hard time right now and you are asking too much of him is co-dependent, IMO, and Laura is doing a great job describing what that is on her thread. I think it is extremely healthy and necessary for you to acknowledge and process your very legitimate feelings of betrayal.
It is a gift to your father to ask him to grow and come to understand that in order for the both of you to have a healthy relationship he must also acknowledge your feelings and actually strive to meet your needs -- that it can't just be you meeting his needs while you ignore your owns. It would be unhealthy for you and for him to continue to allow your mother to abuse the whole family while everyone, as you've said, ignored the pink elephant in the room.
The greatest gift you can give yourself and your father is to embrace the truth and act on it, not out of punishment or a need to control, but out of love -- out of a desire to strive to build a real, healthy relationship with your father not one that is steeped in lies.
Healthy people who truly love one another in an altruistic, non-codependent way speak the truth in love, ask for their needs to be met without fear (that the other will go away, that the other will punish them, that the other will attack them, that the other will blame or shame them); they express their desires with a feeling of safety, are accepted for who they are, with all of their faults, with both striving together to always treat the other with love and respect and when one fails the other, being open enough, humble enough to be accountable, admitting their error and fault and then doing something about it -- it just isn't enough to say I'm sorry, I can't, IMO.
Hope this helps in some way bring you peace and clarity in regards to what a healthy relationship between a father and daughter could be -- now I guess the question for you to decide is whether that is what you want.
You have the right to want that. We all do.