I have SD63, OSS61 and YSS57.  OSS is a prince,  we've always gotten along.  Nowadays, he cuts our grass and does minor repairs here for DH87 and me79. He brings us eggs from his chickens.   For awhile, he and his wife were visiting my widowed mother and disabled sister regularly, a compassionate act.  I know he did that, in part, for my sake.

I wish I knew why our relationship went so well when the other 2 didnt.  SD has always been jealous and YSS had the BM loyalty bind.  OSS loved BM, too, and was often there in her final days.

It's nothing I did differently with him, we just jelled.  He's a kind, humble, thoughtful person.

Of the 3 adults, i have a good relationship with one. It's not the adult SD, though. The difference is him, he's respectful and doesn't cause any drama. He lives with my SO full time at age 20 and is really no trouble. The SDs on the other hand have exhibited out of control behavior issues. I don't see how there could be anything you could do in the face of disrespect, mental illness, etc. These SDs live out of state but any visit or even phone call contains crying, screaming, showing of private parts, and demands for money. WTF can you do with that but avoid it? 

I met my SS-32 when his mom and I met and started dating when he was 15mos old.  We married the week before he turned 2yo.  I raised him as my own.  He asked me to adopt him when he was 22.  We made that happen.

A couple, or more accurately a few, specific things that helped....

1.  His mom and I agreed from early in our marriage that if we were going to be equity life partners we were also equity parents to any children in our marriage. As it turned out, SS is our only.  I raised him as my own.

2.  Standards of behavior and standards of performance were set very early and evolved as he grew up. We kept them age appropriate. That included the requirement that he be respectful of his mom and me, and of his SpermClan.  The SpermCLan part was our biggest challenge.  They were liars, manipulative, toxic, and committed to PASing SS though my DW had full physical and legal custody and they had only limited long distance visitation.

I'm not sure what they were thinking the PAS thing.  Effective PAS is a CP thing far more than it is an NCP thing since CPs have the majority of the time with the kid(s).

3.  Facts. We never lied to him, he knew the facts in an age appropriate manner starting in his later single digit years.  Usually in response to his questions about their lies.  Ultimately he knew it all.  Which prepared him to protect himself in real time when he was in SpermLand for visitation and ultimately as an adult.  People like that do not stop their toxic manipulation just because a date on a CO ends the official oversight of the life a COD (though DW and the SpermIdiot were never married)..  Ultimately their behaviors and his knowledge of the facts ended his relationship with them.  Not our intent, but... them being uninvolved in his adult life keeps their affiliated drama in his past rather in his present or future.

Standards and boundaries are the framework of successful blended marriages and successful SParent/SKid relationships.

IMHO and experience of course.

I have one SS and we get along great. I first met him when he was 6 and I had older sons in HS. I think we did okay because I stepped back a lot. Poor kid was already reeling from a nasty divorce and had his mom in his ear . After a bit we slowly introduced me into the picture. If we did something one day of his weekend, I made sure to let it be the two of them for the other day. To be fair though, in spite of her nasty, his mom raised a decent child. Now he's almost 30 and he and his fince just bought a house in our town and we see them quite often.