I agree that they're taking this too far. It's detrimental to THEIR mental and physical health to carry this resentment all this time, and they're certainly not teaching their children very Christian values. By carrying resentments, they really hurt only themselves, as there is a saying "resentment is the poison I take to kill you." And it's true. Resentment are like cancer -- they eat you from the inside. Granted, I know this hurts you too, but believe me they are suffering more.
I really don't know what you can do. Have you sat them down and had a long talk with them (without your partner present)?? Perhaps it would be a good idea to do it again, if you've already done it, and remind them that they've accepted their father's new partner -- how is it fair that they won't accept yours? Forget the circumstances of how everyone got together -- you both have new partners and everyone should be accepted! They are adults now, and they're acting like children. It's almost like they're at school playing games and picking who is going to be on their team -- but your partner is the one who is excluded. How childish.
Tell them to grow up. And don't accommodate them anymore -- always take your partner to family gatherings, just like you SHOULD. Either that or just refuse to go. It's your choice. God bless you both, and please take care. Hugs, Lee