thebin:One doesn't have to make one's self miserable at a wedding (by inviting a person you have a right to detest) for the sake of manners. I think since the LW comes into play in this family wedding ONLY as the other woman- not as any sort of a step-mom, the "manners" of her normally automatic invite are null and void in the face of her role in a far more weighty and permanent indiscretion.
I disagree with this only because she isn't just "the other woman", shes his wife. And it wasn't 6 months ago, it was 5 years.
I agree that you shouldn't have people at a wedding that make you miserable, BUT I think it's wrong to invite one half of a unit of marriage to a party.
The daughter had 2 choices
1) Invite them both
2) Invite neither
Inviting the half of the MARRIED couple that she wanted and not the half she didn't is being selfish. You're right, she's entitled to not have people there that make her miserable. But if her father's wife makes her that miserable she has the option to not invite her dad. It sucks, but you can't have everything you want, all the time. She has to pick if it will make her more miserable to not have her dad or to have his wife. Then pick option 1 or 2.
The dad is now in a lose-lose situation, and like I said, should make the best of it by skipping the party and attending the ceremony. He sees her get married, but also sends the not-so-subtle message that his wife is part of his life, and if they want in to that life more than just a little that they will one day need to deal with that.