J.B.
I think that we've got the same stepmom although I refer to her as the stepmonster or my dad's wife! She is wife #4, my mom being #1. I won't go into details on things that she's done, but my brother's family and me and my family refuse to have anything to do with her, period! She is just an awful person and my father has pretty much lost his family due to her. He's an only child so we are all the family he's got. My son is 30 months old and I think that he's seen my dad maybe 6 times in his life; he has no clue as to who he is when he does come around. I love my father too, but if he will allow her to treat us the way that she does, I don't have to subject myself of my family to it.
I saw that you stated that he's your only parent left and I'm so sorry to hear that, but you need to decide is the relationship that you have with him because of her worth it? You've just got to decide what's best for you. I know at first we told my dad that we want to have a relationship with him, but not if it includes having her too. We respect that he loves (at least we think he does, but don't know why) her, but we don't have to deal with her, nor will we. If he wants to see his grandkids (4 total), then he will do it on his own because they will not be taken anywhere she is and she is not welcome in any of our homes nor will we come to theirs if she is there.
If you ever need to vent, PM me and we can compare notes. Good luck in the decision that you make. I know that it's a hard, heart wrenching one.
~J.