I was disengaged from SD because she was over 18 and not living in our home. I was partially disengaged from SS in that I took care of all his basic needs (food, expenses, clothing, etc.) but I had no hand in discipline or rules when it came to him. In my case it was a disaster: the kid was out of control because no one set any boundaries or disciplining him. I would never recommend disengaging from a skid who is in your home full time.

If they are adults and completely out of your home there is no reason for contact. If DH needs to see them, let him. Outside of your home and presence. That also means no talking to you about them. You dont need to know. Disengaging from SD meant I wanted nothing to do with her. My STBXH couldnt understand why I would not let pregnant SD and her entourage (junkie boyfriend and dog) move into MY home when BF's mother could no longer stand to be around any of them.

If you disengage from adults, do it totally to the point of non-existence. 

DH is estranged from 3 of the 4 skids.  First one went when he decided that he didn't want to follow our house rules.  (attacking his step sister wasn't allowed.)  Second and third one went after they attacked DH and broke his shoulder.  The fourth one is graduating and we stay in touch with her.  I even got a Mother's Day text from her.  (before mine even got around to it! )  

We worked with a therapist for 5years and were lucky to have one that had been aliented from his own kids.  His advice was to love the ones your with and give what you get.  Its worked for us.  If the first three want to reach out, they can.  They are in their 20's, more than old enough to make an effort.  I will tell you that it can be done as long as you put the behavior on the person who should own it.  The other kid needs to be relieved of any guilt and repsponsibility for the situation.  The removal of that hot mess of a SS will be astounding for your stress levels and life quality.  It was for us.  

He first made the decision to break with the younger one, then I stepped back from the elder six years later. Without me acting as a buffer and doing all the emotional labor, it did not take my DH long to realize what a vicious, narcissistic tw@t his eldest kid actually is, and he also disengaged. During those six years between disengagements, we had regular interactions with the elder skid, who supported our disengagement from Younger Skid. But after falling out with Elder Skid, these half-siblings, who showed zero interest in each other growing up, soon formed a tight "let's hate Dad and SM together" club.

Here's the thing to remember: loyalties in dysfunctional families are fluid, fickle, and unpredictable. The only constant is that you, the SM, are an outsider and an easy target. So, take the current situation with a grain of salt; be cordial, but don't say/discuss anything skid related with anyone else in your DH's family; and strive to maintain emotional distance from these individuals related to the man you married.

My wife is generally much closer to my family than her own.  She does remain close to her own family but the disparity in life experience between my bride and her own family and the very similar life experiences with my family have set the stage for this situation.

We visit my family regularly together and she visits with my parents several times per year when I am not available to go.  I make it a point to keep her focused on seeing her own family several times per year just to avoid any issues that may crop up in the future.  Usually I am working during those visits but I do make it a point to go on an IL clan visit/outing every 2-3 years.

Our kid (my SS-25) is awesome and the three of us remain very close.  He is well into doing his own thing and doesn't call, answer his phone or visit as much as his mom and I would like... but that I suppose is the world that Expat parents create and have to live with when it comes to their kids.  While the relationships can be very close and strong, adult kids who are raised internationally and/or with regular major moves tend to live their lives far from their parents.

After years of nastiness, accusations and disgusting man baby behavior, I am completely disengaged from ss32, DH can see him all he wants but I have zero interest. If I must be in his vicinity I will say a polite "hello" and that's it.  ss26 is alright and he and his family are enjoyable to be around. I've semi disenged from them meaning that DH is in charge of the relationship w/ss and his grands.  DH is also in charge of shopping for them, entertaining and all communication. I'll stay and visit if I have time.