Perhaps your father chose to spend money on her - as his spouse he owed it to her too.

Did you and he talk about and agree to an understanding of expectations regarding things like education, etc?

Regardless, that doesn't diminish the feelings and resentment you have for her. No one I hope is saying or expecting that you be her friend or even an aquaintance. Talk to your dad and have that conversation now about expectations. Let him know you would love to spend time with him but would prefer to do so without her. But, understand that he has a choice too.