I feel so sorry for your daughter to be in that situation, your wife is making the message very clear to your daughter: she's not the same as the bio kids and never will be.

That's so hurtful (my parents broke up when i was a kid and i've had step parents, it would have broke my heart if that ever hapenned) and mean.

I think you should say to her that it's fine, but you won't attend, after all, if she's not a "real" mom to your daughter, then you're not a "real" dad to her kids so why should you be there to celebrate with them?! And go spend time with your daughter, have a day out, enjoy.

And make sure to not invite your  your wife or her kids for Father's day, remember, they're not your kids!