Aw, I'm sorry
People can be so hard to read sometimes, it's not your fault. Just call the ex wife and tell her the same thing you posted here, and I'm sure that she will understand. It's hard to deal with divorce and exes, and who to invite to family gatherings, etc. It's no wonder you didn't know the protocol, because there really ISN'T one
If she's not psycho, invite her and see what she says. Smoothing things over with her should bring everyone else around.