Next time you hear, "You're the mum, you're supposed to be the one in charge," remind them, "Yes, I AM the mum, YOU need to back off and let me parent difficult child in my own way. Stop interfering, you are NOT his parent, I am. Back off and do it my way."
The thing is, what works with our difficult children (Explosive Child style) is often the opposite of how people expect us to parent. And what works for one kid can often backfire with another, so you need to get it across to your PCs, "This is what we are doing with difficult child. Respect my ways or else I'll remember all the old, traditional, 'spare the rod' techniques and use them on you."
I have written a summary of Explosive Child (it helped me get the info hammered into my brain thoroughly) and worked trough it with my other kids. They took some time to take it on board and yes, there have been times when what they had to tell me about my parenting, while unpleasant, was actually constructive and helpful. So they know I will listen to them. But I WILL NOT LET THEM UNDERMINE ME.
I will support them also, but if they undermine me in front of difficult child 3, then they will in turn find themselves undermined and criticised in front of him.
I do strongly suggest you STOP expecting difficult child to be adaptable (since when has he been adaptable?) and instead put the onus on your older PCs (who should be old enough to be malleable, should also be mature enough to see the need for gentle handling) to be the ones to make the adjustments.
Point out to them that getting difficult child to be adaptable is going to take time and patience. It cannot be forced. You have learned a lot over the years, just because you didn't know all the answers when they were kids doesn't give them the right to make difficult child as miserable as they felt they were. This isn't about retribution or justice, it's about compassion for a brother whose world is much scarier for him than theirs was for them.
Give them a quick lesson about baskets and about choosing battles. They need to be adaptable; they need to be the heroes and make the change, even before it's been asked. They're now the invaders in difficult child's world and they need to give way to him. it is NOT spoiling, it is instead paving the way to him feeling safer, more confident and therefore more able to slowly become more adaptable.
Of course it's not fair to the PCs. It's not fair for you, either, to have to police all this and ride everyone so closely. But them giving difficult child a hard time doesn't make it one bit more fair for them. It only makes trouble for everyone, will make you cranky with them and will make everyone's holiday miserable as a result.
All the best with it. If you want my review, PM me and I'll send it to you, so you can print it out for them to read. I doubt they'll take the time to read the whole book.
I actually found a very brief summary of the baskets, and stuck it up in our family learning centre (AKA behind the toilet door). It then eventually got the message across to them. They also began to see it in action.
Also as I began to use it on difficult child 3, what happened FAST was that difficult child 3 behaved much better for me, but became even worse for anyone riding him hard (ie anyone refusing to get on the page with Explosive Child). This taught them FAST to do it my way or to shut up. I actually had to say at one point, "Shut up, do not interact with him if he is difficult, leave the room and let me handle it. If you do not, he will quickly learn to argue with you on sight."
For tomorrow, try to plan the day for people, to allow difficult child space to be where he wants and to do what he wants, in ways that will not impact on the PCs. And set it up with the PCs to leave difficult child alone and to give him space and time. The issues where they need to give him space -
1) Do not change what he is used to; do not impose your will on him but instead, adapt to work around him.
2) If you must interact with him, give him time to adapt. Do not force a sudden change, but give him time to get used to it. For example, if you want him to have his bath, give him a time warning. "I need you to have your bath in half an hour. Please get your computer game to a point where you can save it, or pause it. You have half an hour to do so." Give him choice, give him control. If you're getting food for him, don't insist he eat something he doesn't like or want. If it really doesn't make any difference, let him choose. Don't make him drink juice if he wants milk. Ask him which he wants. It's common courtesy.
3) Speaking of courtesy, he learns it differently. Even if you feel he is being rude to you, do not react. Instead, treat him with courtesy at all times. If he offends you, leave the room and do not interact with him. But do not shout at him or get on your high horse for being impolite. He needs you to demonstrate to him, by your manner to him, how he should behave towards you. So if you're impatient with him or shout at him, don't complain if he is shouting at you or impolite or impatient with you. You're older, you set him the good example he needs.
4) I don't care if you think this is unfair; of course you are right but unfair doesn't matter. All that matters this month is what is working, this month. Next month the rules change. But now is now.
This works. It requires patience and consistency. Those not willing to get on board will be the ones to pay the price. If the PCs fail to follow your requests, then they are showing disrespect to your wishes as a parent, and in doing so are demonstrating that they are right, you are not in charge. You are not in charge OF THEM, and they owe you a huge apology for their criticism of you, because they are NOT helping one iota, a failure to obey your wishes in this is only making your task more difficult.
And you don't need that. Not from your alleged easy child kids.
If you need to, feel free to show them this post of mine. And tell them that my PCs (I include easy child 2/difficult child 2 here) also had difficulty with this one, and found out the hard way that they had to do it my way, or they would be the ones to lose out.
Life isn't fair. Whoever handed difficult child the neurological grab bag he's stuck with, wasn't playing fair. If the PCs are hard on difficult child because he isn't a easy child, THEY aren't being fair to HIM. They're treating him badly, as if they're punishing a blind child for needing someone to hold their hand while they try to cross the street.
Good luck with this one.
Marg
But you really have to stay calm throughout this.) When I mix up my errands, I try to choose places with-drive-throughs, so the meltdown provides a minimum of outside involvement. Try to involve as few people as possible. 
