Clearly this is a tough situation for everyone really.  The guy's mom has clearly laid the groundwork that got them to this place.  Her son feels he has to carefully craft it so that his mom doesn't have any hint that his dad's wife is in a "mother" position.. and doesn't take the smallest blush of attention from her role in that space.

He is damned if he does.. damned if he doesn't.  If he were to go full on inclusive with his dad's wife.. he gets punished by mom... he excludes his stepmom.. or marginalizes her inclusion? the stakes are lower... because you can more easily rationalize that "yes.. she is not actually his mother.. so we can draw some guidelines."

At least the guy was trying to get ahead of things by setting expectations ahead of time.. gotta give him small credit for having the nutellas for that I guess.

I've been disengaged from DHs kids for near 5 years. Not seen or talked with them. I got a birthday invite for one of their kids and I got a wedding invite during that time frame. As I said in another post, both invites happened only because my DH would NOT attend anything I wasn't invited to with him. As for those stipulations, that would be a hard NO for my DH also so that's probably why I didn't get what that woman got as for as directives and demands.I opted out of both events. I made myself happy and I'm sure, made DHs kids happy. They could then continue on with their fantasy beliefs and get DH and their BM in some pics. 

If I were to advise this woman, I'd stay home with my kids, I'd shut DHs family down simply by telling the truth. The invite is fake...he's clearly stated to X he doesn't even want me there. YOu have a problem with that? Talk with your groom relative and keep me out of it. I'm giving him his wishes. 

"...after my husband asked him why he was making such a big deal out of this, {SS} said I needed to know my place and then admitted he didn't even want me there."

This adult son had the gall to tell his FATHER that his father's wife of 25 years was not welcome? There is no excuse nor credit to be given to a so-called grown man for being so gutless and brainwashed to tell a grown woman she "needs to know her place."  

If I had been DH I would have said, "Son, my wife's place is beside me. If you do not want to include her in your wedding that is entirely your choice.  My choice is that neither my wife nor I will be there.  I send you and your bride all my best wishes and hope that no one ever tries to divide you as you've tried to divide us."

And kids of new marriage and all are included in family wedding pics. 
 

the fact this ss felt he had to emphasise and put stepmum in her place etc just shows what a petty vindictive little shi* he is. He's go grudges against daddy because he remarried someone else.

hypocrisy is in alot of these scenarios, biomum can bring her 10th recent bf since divorce to wedding but daddies wife of several decades that he has kids with is banished and treated like dirt.

the dad is put in a loyalty bind to choose first family over his wife, wife caves in as doesn't want drama and tells her husband to attend but how happy is he that his wife of several decades and kids (half siblings of the groom) are not invited. Such a miserable depressing event to attend and weddings are meant to be happy places to celebrate family and love/unity, not bring up petty drama