[Mokka]

my advise... for what it's worth?

If you try to confront your dad before the wedding, what outcome are you looking for??
I think it is highly unlikely that your dad will stop the wedding at this point, or that he will see the girlfriend in the same way you see her. If he hasn't realized it yet, he is not ready to see the truth. Denial is a powerful thing.

Is your sister old enough to fight her own battles regarding the house rules and living with the gf and dad? If she were 12 and under, I would say then it is within your rights to jump to her defense, but if she is able to give your dad a first hand account of the situation and how it is affecting her, then you need to support her in having those hard talks with your dad and not try to fight her battles.

In regards to the gf saying horrible things about you, as long as it isn't changing anyone's opinion of you, like your sister, dad, or any other family member, don't rise to it. The gf is probably trying to get you to kick off and cause an argument with your dad so that she has more ammunition against you. Name calling and accusations suck, but if they are not believed by those around you that you care about, they can't really harm you.

It sounds like you have already had conversations with your dad about the gf, and that your perspective has not been received well. It will not go any better for you to confront him the day before the wedding, and it may do irreparable damage to your relationship with your dad.

So I guess my opinion is you shouldn't confront him the day before the wedding. It will probably get worse when they are married yes, but your dad is an adult who is allowed to make decisions for himself, even if they are not great decisions. You also get to make your own adult decisions, and part of that is realizing, we can't control another person's actions, only our own.