In my experience being a stepdad of 3 kids, the biological children already have a hate towards the step parent. They direct their anger that way, cause tension and undue stress. This is well documented behaviour. Look it up. The step parent has the added stress of being expected to like you no matter what. You don't have that pressure and obviously don't.

Now maybe you are extremely pleasant and treat her with the upmost respect. Doubt it. But it is normal to have these feelings of blame and hate. Now is the time to self reflect and try to make things better. It definitely doesn't help your dad knowing 2 people he cares about can't get along. He deserves to have a relationship too.

I suppose its possible that she is an asshole. Think of her interactions with other people. Does everyone hate her? Does she have friends? You might be the exception to the situation but it's likely more the situation than the person.