Kate has been excluded from all of their major milestone celebrations: birthdays, graduations, etcetera

This guy is a real prize (NOT). My guess is the only reason he is upset about the wedding is that people will notice he is there without his wife. When they ask about her, he will either have to lie (sudden sickness most likely) or tell the truth (and he/precious daughter will be seen in a bad light). He's not concerned about his wife and younger kids. He's concerned about himself.

The only part I agree with is that daddy set a precedent a long time ago letting Kate be excluded. Why would he expect it to change now? Daddy should have found his balls 10 years ago. The rest I do not agree with at all.
Hope this man in not contributing financially to this wedding, would also be like paying people to be rude to your wife. 

I agree that Dad should not have originally allowed Kate to be excluded - this was enabling of the excluding behaviour by his adult kids and let them think this was OK by him.  I thought the advice given was stupid, and almost 100% likely not to work - too little, too late.   If I were him I would either not go, or take Kate along in spite of them trying to bar her.  What a twunt.  

This, ladies and gentlemen, is what you get when you spoil your children.  Hope dad is enjoying the product of what he and BM created.  Kate should run.

What a winner this guy is! 

then afterwards invite both his daughters from his 1st marriage and tell them to grow the eff up

He should have done this ^^^^ a long time ago. Why should they grow up, now? They have gotten away with acting like this for years and he went along with it.

I don't care if they have ever liked her or not, he should have taught them some manners. They don't have to like her but they need to respect the fact that she is his wife.

Say you cannot force her to invite your wife and her half siblings, but as she herself is creating a new family, it would be a lovely gesture of reconciliation on her part. If she refuses, attend by yourself. You want to be able to have a connection with your future grandchildren, however tenuous.

And this little tidbit ^^^^. I disagree with this. She is basically saying, do what she wants if you want to see your future grandchildren.

It just gives the daughter the green light to use the grandkids as weapons.

I wonder if he went alone. Maybe he did and Kate wised up, and he's alone, now!

My Dh was married before me to a woman who had 2 children. When they divorced, his stepdaughter maintained a relationship with him and a bedroom in his home up until we met, then shortly afterwards moved to Manhattan to nanny for a family.

When she got married, the invitation was addressed to him only, even though we had met multiple times. No plus one for his wife. We had met multiple times and had cordial dialogue and I didn't understand the reasoning behind it. 

Honestly, was happy to not have been invited. No need to spend money and waste a perfectly good day off at an event that was indirectly a result of my husband's ex's vagina... 

My  DH wasn't happy and called her to question it. She told that things were tight and they just couldn't get me in, although my younger sd's new boyfriend of one month was invited. He told her that we weren't going and also called my in-laws who had also all received couple invitations and they didn't go either...

She is no longer included in any family gatherings because while they embraced her as family when she was a child even though she wasn't related, she wasn't able to do to even be cordial to me even though my husband essentially gave her a home. She chose to pull an ultimatum and now has to live with that choice.

If I was the father I would show up to the wedding service with my wife and my children. Then I would leave. I wouldn't attend the reception. The Church doesn't have a head count. If the "Bride" didn't want my wife to sit in that first row for immediate family, I would take my seat in the back with my family. I would make sure that Juuusssttt enough people know what is really happening before I left so that there would be gossip to mess up her perfect day. 

As regards that "advice". Totally off base. DH wouldnt let SD exclude me from high school graduation, he didnt want to go alone. I ended up going.

I think that we all agree that leaving the SM at home while the bio dad continues to "have a relationship and choose the children first", is totally wrong and definitely shows how general society views the new family paradigm.

would never, ever consider pandering to bad behavior the way that bio dad has. He respects me and values our marriage too much.

And I agree, Prudie's advice sucks. It seems to her, some marriages are more important than others. She also chooses to disregard the vows the LW made to his wife, just so he can watch his vicious daughter make them to the person she loves. She essentially encourages the letter writer to be a doormat for his older daughters, just in case they might one day spawn.

So ridiculous.

I myself never participate to important events of DH's boy, the distance is being helpful. DH is going on vacation alone with this kid, he visits this kid alone, I do not intervene or try to alienate the dad from the boy but I do not want to be a part of this relationship. 
 

I see the boy something like once a year and do not miss him at all, to be honest.
 

I would not expect him to invite me to his wedding or any other important venue. I would feel uncomfortable if he would invite me, in fact.