I'm a Mum & a StepMum, DH is both a Dad here, a DSDad, and the NRP to his DS. Honestly OP, I've seen all sides of this so please do listen without jumping on me too.
My advice is to pick your battles. Is this one really worth it?
Is your DP really incapable of pulling a 24hr shift to ensure the safety of his own children, one having SEN? For one day?
If that was my DH I'm sure he could manage.
But if he really couldn't due to the nature of physically demanding work, I'd willingly alter my own plans so that I could help him out with DSS, and ensure his DCs safety.
Because I love him, and because that is DH's child!
And sometimes life is unpredictable, and fucking infuriating when it comes to plans falling through.
My own XP let me down many times. But DH would never have left me to struggle alone with my DS and compromise his safety, because he was bitter about my XP ruining his day.
Why would you leave your DH struggling, if he really will be that tired?
From here, it sounds like you simply don't want to help the person you love, because it would have the side effect of benefitting his XP. That isn't healthy.
You mentioned his family all back you up, that his XP is an unreliable, part-time selfish, piss taking parent.
But you still chose to be with him despite knowing that!
So surely you went into this with open eyes, and didn't expect it to be all easy? You obviously thought he was worth it.
And his DCs don't deserve to suffer, nor should they be made to.
As for mentioning his stepson as "HER child", clearly he doesn't see it that way, or he simply wouldn't have maintained contact with him. They are his children. He clearly cares for them both, or he'd take the hard line and only have contact with the one who is biologically 'his'. Think carefully whether this is for you.
Because when it comes down to it, if you try and make obstacles, it will be you shown the door, not them.