Easy. Leave your partner.

Trying to get rid of someone’s children is horrific. No matter how much you dislike the skid you shouldn’t be trying to get rid of them, I sincerely hope this is a joke. Do yourself & your partner a favour & find someone else.

Why would you want to get rid of them skds? 

I understand your frustration, and I've felt the same in the past; even made a similar blog. But trying to get rid of skids is only going to hurt your partner and harm your relationship with him, and most likely destroy it. I hate my skid but I'd never stand in the way of DH maintaining a relationship with her. You have the right to have a basic level of rules and standards in your home, but parental alienation is not the answer. 

Are you the one who thinks SS has sexually abused your non-verbal LO but has nothing official to prove it? So even if you leave you can't prevent your child from being with SS?  If so, I'd watch SS like a hawk to prevent him from harming my kids and look for additional offenses. I'd probably also get my ducks in a row to leave, because short of this kid going to juvie, that's the only way you'll get rid of a kid your H has full-time.

Keep the facts of toxic Skid behavior front and center for your SO.  As  you build the inertia for booting the toxic spawn start salting the facts with suggested solutions.  As SO starts to gain clarity start ramping up the tension by turning Skid behavioral issues into bigger deals.

Then start pointing out any criminal behavior that might motivate you to call the police on the toxic pelvic projectiles.

Then make the call when they do the thing that is the straw that breaks the camel's back.

Military School is a good option.  These schools have more than a century of experience in dealing with incorigible little shits.  Young cadet leaders can readily chew up and spit out ill behaved spawn of failed parents.  Only they spit out well behaved, polite, respectful young adults where failed parents just grow larger and more intolerable children who have little chance of reaching viable adulthood.

Just my thoughs of course.

I wouldn't let that kid alone with mine. OP can't you talk to your husband about ensuring the mom takes him 50% of the time? He sounds awful and it's destroying your own kids who you have a duty to protect. Nothing funny about it, my kids would come first.  Boarding school????

The easiest way that doesn't make you appear to be a complete monster?

Just... don't care. Disengage. Don't help.

When the kid's of age they'll probably choose the other parent.

I can see the skid going to a place where her crazy ass behavior is tolerated, and she isn't used as an example of how not to act. It also removes a LOAD of stress from you.

Ignore. Focus on yourself or your own children. Don't do the laundry, don't spend your money, don't sacrifice your time in the car, don't babysit. It's not your responsiblity, and chances are, the stepkid will pick up on it, especially if you live with "ours" babies, and go to the house where they are allowed to do as they please.

Assuming you have stepkids who aren't being raised correctly.

Leave your partner. I did and I am so much better off now that a few months have passed. Those kids aren't going anywhere. Btw my ex downgraded big time immediately after I left him. It made me realize he was just using me all along. No regrets!