Does she have a kid that is the half sibling of your SS or maybe some ex stepsiblings? If so then it's not that weird and she's doing it for the kids' relationships. Annoying for sure but not totally out of line. 

If not then it's very weird. I would then assume she is a lonely woman with no family. Maybe some step moms would like to stay in contact with their step kids after their marriage ends but I am not one of those people! 

Madam,  I am not sure why this should be of concern to you.  If the marriage ended decades ago, then this child is a grown adult and can have any friends he wants to have.  Believe it or not, but marriages end for lots of reasons, sometimes those reasons have nothing to do with the stepkids.  So perhaps this ex-SM had a good relationship with the kids and have both decided to maintain it.  Weird or not, this is a relationship between 2 adults, neither of which is you.

I still talk to my stepson. When I ended the relationship with his father my only guilt was being another woman to come into his life only to walk away. His father jumps from relationship to relationship and is a narc. 

SS had a very unstable upbringing. That being said he had every right to just dismiss me knowing what I didn't know at the time that his father is not a one woman kind of guy. But he didn't and we had a great relationship. He was always so kind and made me feel like part of the family. 

So when I ended my relationship with his father I made sure not to treat him any differently than I always had because he had nothing to do with it. Plus he always liked being able to come to me for advice. 

My current SO also still maintains a relationship with BMs oldest daughter. She has had such an abusive life and appreciates that SO was so kind and caring to her when she was younger. 

She comes to visit us and we invite her to do things with us sometimes even now that SKs aren't here anymore.

I think it's a good thing. If they're still maintaining a relationship they must have been close during the marriage. Why should their relationship need to end because the marriage did?

My husband has ongoing relationships with both the stepmother and stepfather who helped raise him, notwithstanding that they're no longer married to his parents. It's a "Facebook, text message, visit on the rare occasions when he's in town, call on birthday" relationship, but that's because they live many hours away.  He's closer to his former SF than to his mom's current husband. 

If it doesn't directly impact you, why do you care?

I had 2 stepbrothers from this union.  My mother was with him dating + married for 8-10 years.  They divorced and my mom remarried. 

We are all still close to this day.  They came to functions and events throughout our lives.  My son is very close to his stepmom even though his dad and she divorced.  It is not at all unusual.  

I think you need to take a good hard look at what is really bothering you.  Could it be that the connection to the ex in your life is bothering you??  I totally get that if if it is but it's something you have to be honest with yourself about and work through yourself because you can't change who other people have relationships with.  If you try you will be the one left out in the cold.  No one wants an ex connected to their life I get that.  

. The SS will try to hide that the ex is coming so we can run into her.  My husband sees it as being disloyal.

SS knows his father wants nothing to do with his ex. But try's to play a game to get them together in the same place,  This is being disloyal to his father.   If SS want SM over more then his father that up to him.  But should tell BF what's going on.

SS May like/ love SM more then his BF. Or feels SM treated him better,   That leaves you with two choices either go and deal with the ex,  or cut off ties with DS   That s personsl choice that DH has to make, 

I have a 26M son that I had very young. Wasn't married to his father. His dad got married when my son was 6 or 7, and his wife was always great to my son. He left her, they hate each other, but she still sees my son, they go to the movies, she invites him to dinner once in awhile. His father is trash, so we have that in common lol. I love that she still wants to be in his life, BUT she and I get a long fine.

having no control over various aspects of it. Which can actually be freeing, once you stop letting other people's problems affect you.

It's natural to feel protective of your spouse, but consider this: the dynamic (which IS certainly dysfunctional) was well in place before you came along. It's not about you, nor does it concern you. Clearly your H has a shite relationship with his toxic son, but that's HIS problem to deal with. We all have to pay our dues for past mistakes, so as long as it doesn't impact you or your finances, you have zero obligation to have anything to do with those people. Playing Happy Family with snakes is a complete waste of time, anyway.

A lot of us married men with kids and set about trying to fix their problematic relationships. Big mistake. Look at it this way - if your H had a problem with a supervisor at work, would you get involved? No, because you recognize he's an adult and those are HIS issues to deal with. Remarriage includes a certain amount of separateness; you have your baggage, he has his, and each is responsible for their own stuff. HIS ex and HIS son are HIS problem. It's okay to feel empathy for your H and be a listening ear, but it's best to strive for detachment and stay away from the drama.