What an awful situation you're in, and it doesn't sound like your Dad is making it easier by letting you in on any marital problems. People can say whatever they want about how you feel about the inheritance, I know my Dad has some family things like the family bible, his old guitar stuff that I would like to stay with kin when he passes and not go to our stepmother and stepbrother. It's human to think along those lines and it doesn't make you a bad person because of it.
All you can do is just be his daughter. Does this woman love your dad? Does she take care of him in a way that he likes? Can you be a part of their life by loving your step-niece/nephew? It does sound like you and your siblings started out not wanting to like her, and went out of your way to see all her faults. We all have faults. Can you find anything about her you like? Can you love her because your dad loves her?
All you can do is just be his daughter. Does this woman love your dad? Does she take care of him in a way that he likes? Can you be a part of their life by loving your step-niece/nephew? It does sound like you and your siblings started out not wanting to like her, and went out of your way to see all her faults. We all have faults. Can you find anything about her you like? Can you love her because your dad loves her?