You could start by having a conversation with your SO about him being the one picking up after his kids. If this goes over smoothly it should theoretically make him start hounding the skids to pick up after themselves..even if he does take the task on he will become tired of picking up after them very quickly.

Keep an area to yourself. Your computer desk, your bedroom what ever you choose. This should be a skid free zone..so there for it is clean. This will be designated to prepare for the possibility that your SO just doesn't give a flying hoot and your home is a disaster. You can spend your free time here for the time being so you can restrain yourself from picking up the messes. I have the "I will just do it because it will be done, I am faster at it, I can do it better" personality so if you are anything like me it is very difficult to let something just SIT..for God knows how long. But you will have to, just to get the point across.

Get the Skids a separate hamper for their dirty clothes. Not sure how old they are but if they are old enough to do laundry..well I guess they have a new task!

When I cook I clean up the pots and pans my plate, fork, knives..etc the skids do their own dishes. My SO does the majority of the direction and "reminding" when it comes to them picking up. I only pipe in when I have heard "I forgot" one too many times.

I still clean my floors, sweep, mop and the usual things but I will not have to pick up skid items so I can do these things.

So basically the key to not being taken advantage of is not letting them take advantage of you. You have your clean space, the rest should be on SO.

I am not a nagger either. And it these situations its a HUGE disadvantage.

I am a strong person and don't consider myself a doormat yet somehow that is what I have become with DH.

I am a person that has an ask once policy, be it at work or here at home. I ask once, if its important to me, I get it done because I have other things that need to be done too and sometimes I just can't wait around forever.

Throw in a personality like DH which some would call lazy to some degree or even self centered - and that makes me a friggin doormat.

I just had to come to terms with a few things that I CANNOT allow myself to expend energy on anymore - which lead to areas of disengagment for me. I am unable to disengage completely - its just not in my personality so I keep getting dragged in but here
are a few I was able to disengage from:

1. skids homework - this doesn't effect me (yet) they don't do it, I don't care, I am not going to waste time on trying to get them to do it. Now if they flunk and never move out, i may regret this.
2. ss7's behaviour - its a losing battle, I have no reinforcements, I limit myself to stopping him from hurting others or doing very dangerous actions that could result in a major injury for himself, otherwise count me out.
3. Reminding DH of things the skids have (like school events) - not my job, his loss, I had no interest in attending anyway.
4. laundry - I ask once, bring your laundry, if its not there, I do not wash it. DH complains skid X has no clean clothing, I remind him I asked once and others brought theirs down I just assumed he\she had nothing - its on them.

But things I can't disengage from:
1. Meals. We all have to eat, I have 2 bios in this house, I can't wait for DH to get home to help me make the meals and I refuse to eat at 830pm every night - so if I want to eat at a decent time, I have to make dinner alone after work each and every day.
2. Minor household repairs - I do them myself, I ask DH twice (I have an ask twice policy here) DH gets all pissy when I perform the fixes myself. I have the ability, out of 'man of the house respect' I ask him twice then I do it myself. Its a priority to ME to have things that I use daily to work.
3. Plans. I can't take DH's well lets see and decide attitude, so I now inform DH of the plans I have already made for a weekend with or without skids - he can join or not.