Back when you were upset and talking about divorce and not being happy being a mother, they took that as you saying bad things about someone in their family. Even though things are better now, it will take them a while to trust that you're really committed to your husband and baby.
To deal with the hurt, remember that the only person you can change is yourself. You'll have to decide if it's worth hanging on to the hurt, or if you'd rather put it aside and be a part of the family again. It sounds like you're beginning to feel like you'd like to be included in things again. Sometimes it's better to be happy than to be right.
I think one other possible way to look at things is that they have reached out to you with the invitation to dinner. It may be expecting too much that they will come to you and apologize...they will probably try to go on as if nothing has happened. And after all this time, that may be the way to go.
I'm not sure what your husband could do to fix things, other than to let them know that you were going through a really hard time before the holidays, and things are much better now.
Things won't get fixed if you don't ever spend time with them again. When you finally do see them again, keep things light, don't bring up personal stuff, and be as pleasant to them as you would be to people you interact with at work. Yes, time has passed, but you haven't taken any opportunities to be with them to establish a new relationship.
They've extended a peace flag by inviting your family to dinner. It may be time to put the past year behind you, and start over with them. You sound like you're in a much better place emotionally than you were a few months ago--maybe it hasn't been such a bad thing that you haven't spent every weekend with them the way you used to.
Start practicing being with them now, for short periods of time, so by the time the summer comes, you won't care who's there.