Another survivor of abuse here. I think it's easy to judge when you aren't the one being asked to reject your parent. Children love their parents unconditionally - they are programmed to in order to survive. Turning 18 or 21 or 25 doesn't magically break the bonds of attachment.
My parents were both abusive in different ways. I think I hate them, but I still spent years trying to maintain a relationship because they're my parents and I haven't got other ones. I have gone no-contact and hardly hear from them. I should be relieved that they've made it so easy but I just feel rejected. I think I keep waiting for them to turn into the people they should have been but aren't.
In situations where abuse is involved, it's tempting to assume stupidity is to blame - isn't it stupid that this person ignored abuse or that person still wants to see their father. It's something much older and deeper than stupidity at work.
This girl wants a father. This is the father she's got. And she's blind to what he's like either because abusers are very good at charming, manipulating and grooming people, or because she has swallowed some of the victim-blaming lies our society tells, or she has swallowed the stereotype of what an abuse victim 'should' act like, or she just doesn't want to accept the reality of the situation.
It is very common for people to react with denial - even in the face of clear evidence. Sexual abuse destroys the beliefs people would like to have about the world. Who wants to live in a world where that is possible and the victim doesn't speak up? Who wants to live in a world where nobody could notice? Chances are this girl doesn't believe her sister. Because she doesn't want to. This sort of thing works out well for everyone except the victims of abuse.
OP, do not go. You have to make a choice here. Your daughter made the wrong one, don't follow suit.