If that isn't a blatant attempt to push your buttons, I don't know what is!
I don't think you need to explain to your step-dad why you would be hurt and/or angry. Believe me, he KNOWS. So does the rest of the family. (Except maybe your daughter, depending on her age.) That was the whole point of it all!
It's all about sending you over the edge. If you get ticked enough to blow up and tell him off, he wins. If you get hurt and wish you could have been there, he wins.
If you act like you hope they had a great time and let it roll off your back, it appears that YOU win. You look like the better sport. But we all know it still rubs the wrong way. Shame on them!
It's all the manipulation game that is taught within this religion of "warped love". Is that real love? Would you ever do anythng like that to your child? Consider the source. A bunch of back-biting adults trying to pass as God's chosen. Tsk, tsk, tsk.
Chin up! YOU are miles above that!