If divorcing your Disney Dad that ignores SS’s behavior and embellishes his neediness is not an option, you can disengage by not physically being there when SS comes by and also doing ABSOLUTELY nothing for him in any capacity. I’d keep myself busy and have H do it all. This may help to alleviate some of the stress. You also don’t have to tell your H what you’re doing. Just stop having contact with SS. Keep a healthy distance from him and set boundaries to protect your sanity. If your H is not willing to make any changes be prepared for a lifetime of this and that it will only become increasingly worse.

There can be many levels of disengagement.  But, to me, the basic underlying theme is that it is something that lives within you.  It isn't a set of specific rules or actions.. but a mindset that frees you from the stress and anguish you experience.

A good start is to learn to live the mantra.. Lord give me the strength to change the things I can and the patience to accept the things I can't.. and the wisdom to know the difference.  There are many things about the non-bio child in your home that you honestly just won't be able to control.  However, there are still boundaries that you can set and should set.

First,  discipline?  That's dad's job.  Unless your life or property are at risk.. or the kid is literally in the jaws of a death/injury situation?  Let dad handle it.  You might step in and interject if you are the only one there.. but to use an example of a rule that you dont' necessarily need to get wound up about.. the kid's clothes..  Now, you can insist that dad require a minimum standard of health code standards for the kid.. but I wouldn't worry about the kids messy room.. just set my line in the sand about leaving dirty dishes/food/drink in his room to attract pests.   Again.. this would be something dad needs to enforce. 

But, nagging to take out the garbage? nagging to do his schoolwork? nagging to go to bed?  ALL dad's responsibility. 

Now, you can insist that the livingroom be "childfree" past 9pm.. and that is directed to your DH to insist upon.

You can pull back from the extras you do FOR the kid.. let dad pack his lunch.. let dad give him rides to his friend's house etc... you don'thave to engage him in conversation.. though I do think it's proper to model polite behavior and not completely ignore his presence and speak cordially.

But, you don't have to care.. about his room, his hair, his grades.

But, also, you set clear expectations with dad.  "gee, worried about the little guy.. grades are really suffering.. if he doesn't catch up soon.. I am worried what kind of future he will have.. because he won't be sleeping inour basement..at age 19).   

Sometimes, it is easy to get caught up in the WIN.. you set your feet in the ground firmly.. and you get more caught up in a war of wills.. vs a reasonable view of the outcome.  I recall an incident where I wanted one of my SD's to eat something.. that she should have liked.. it was similar to other stuff she already would eat.. but we got into a chicken fight over it basically.. I was going to MAKE her..and it would ge good for her.. she refused and basically had no way of gracefully giving in either.. We still joke about that incident to this day.  It was stupid and I was the adult.. i let my ego of being "in charge" override my empathy and made it a power struggle.. and why do that?  It just drives you nuts.

So.. it doesn't always mean you won't do things for or with the kid.. but you may not go out of your way.  If you do give rides it's because it is helpful to your husband to do so.. the favor is for him.. not the child etc.. But you don't take on the responsibiltiy in your mind.. and let go of the feeling that you should be in control.. because often times you can't be.   (though you can insist dad stick to a budget etc..... you can also choose to be absent from the home if your DH ignores you while son is there.. find other things to do)

Honestly, distrespectful brattiness that is being ignored by the parent, is not something I could disengage from, nor would I really want to, in my own home.

I'm afraid I would lose it on them both.

I disengaged about three years ago.  I just could not take SS and SD's constant disrespect and dealing with a husband who did nothing to stop it.  I disengaged quietly.  First, by only using my cell phone.  So if they happened to call the home phone, I didn't know or care.  Then, I quit going to visit them.  Since they are out of town it was easier for me.  Then, I stopped any birthday cards etc.  If my husband wants them to have a card he can buy it himself.  No more Christmas presents and I stopped asking about them.  Disengagement really has to happen quietly and gradually.  Eventually, they will get it.  My SKIDS are adults so again, much easier for me.  But you can stop too.  Just be busier without a lot of time to be the unpaid driver, unpaid cook, unpaid maid etc.  The trick is to lose the anger and focus on yourself.  Believe me, your Disney Dad Husband won't rock the boat.  They never do.  It took me a long time to realize that the sky won't fall if I ignored his kids.  If he really needs you to do a favor, do it, but ask for something in exchange.  If he needs you to drive a kid somewhere, counter back by saying "if I do this for you, then I could reallu use a night out with friends" or something else that sets a boundry between you and his kids.  Make it clear the favor is for him and not the kids.  Once his kids realize they can't use you and that you really don't care they will back off.

The main idea is to leave the parenting to the parents. You don't need to be home or be involved in any way when skids come over. I don't do any parenting duties for my SD - DH was responsible for getting her up and to school, home from school, to doctor's appointments, etc. Anything you do is only because you want to (like if I see a cute outfit and grab it, because I want to not because its my job to buy SD clothes). It's hard when they are there so often. Disengaging doesn't mean you have to tolerate disrespect from kids in your house though - your DH needs to grow a pair.