@JillianRae,
Only you can decide whether it's "right" or "wrong". I would say, though, that to me, personally, I think your husband's got the right idea. It was awful, yes, but at some point, the Statute of Limitations runs.
Since you don't mention how your mother feels about all of this, that makes me wonder - is she alive? Have you not told her? Or just not asked her what she thinks? I think choosing between parents is a lousy thing to have to do and if I were you I wouldn't feel comfortable if you were holding out the olive branch and then your mother was insisting that you snatch it back.
So I am going to go with your mother either being no longer alive or no longer caring one way or the other. If that's the case, then I'd suggest that at some point in time, sit down with your father. This isn't a screaming fight or anything. But tell him -
this was hard. It hurt me and my sister, and our mother. I am willing to get past it all because I want to have a future relationship with you, and I can see that your relationship with your second wife is long term and not just some flash in the pan. But I want you to know that there is still pain there. I don't want it to be unacknowledged.
And then sit back and listen to whatever he says. He might get defensive. He might try to explain himself. He might reveal not so nice things about your mother (which may or may not be accurate or unexaggerated, I might add). Or he might ask for your forgiveness.
I think this will solve a few things - you wouldn't just be going along to get along and sweeping the older problems under the rug. You would also be allowing that your stepmother is someone important to your father.
You've got a different family, to be sure (although not so different these days). You need to navigate a few rocky inlets. But I think the main idea is that you and your father are trying to have a relationship again. To give it more solid underpinnings, I'd suggest addressing this now, rather than later (and maybe with the help of a qualified therapist).