Ugh - that's a tough one.
My huge gut reaction upon just reading the headline was "absolutely not!" You can't use one kid to their detriment, as a means to an end for the other kid. My whole being revolts at the very thought.
But of course, your situation is way more complicated than that. So here's my two cents worth. As the parent, you have a lot more info, perspective and wisdom about the big picture and the long term. If next year for your younger is likely to go downhill as quickly and as badly as you say, then it's your responsibility as a parent to make the best decision for your child - a decision a five year old is definitely not equipped to make.
So the question then, really, is how confident are you of your assessment of what's probably going to happen to DS next year? If you dig deep down and painfully scrutinize your own motives, are you confident that you are assessing clearly, and not retroactively justifying the move? If DD wasn't in the picture, and you just looked at where you'd want DS next year, would you still want to move him anyways? There's a big difference between he doesn't *want* to move, and he doesn't *need* to move. In the former case, I think parental expertise over-rules. In the latter, child preference becomes much more relevant.
So then the question becomes: is it in this child's best interest to move, and the actual problem is just that he doesn't know it yet? That's a pretty simple problem to address, and an easy decision. Or is the core question that moving is a more neutral matter, and it's really more about his sister's best interest than his own? That's a harder question. Reading your description, it's not clear if you're confident in your own mind that the move is best for him for his own sake, so I think that's where you have to start. You may need more confidence that you know what the real question is.
Despite my visceral initial reaction above, though, the reality is families have to make these kinds of choices all the time. They're just usually not presented quite as starkly as your decision here. Every time we move a kid out of their standard pathway and/ or neighbourhood school, it has impacts on the whole family. There's tons of opportunity cost, such as parent time lost to other children for transporting the moved one; loss of access to after school activities and playing; reduced family income to spend on other things, etc etc. Every time we move one kid, we have to think about both the benefits and the costs, and how those distribute, which is rarely "fairly" or evenly, particularly in the moment. The best I have been able to do is try to make sure it balances over the long term, and help my living-in-the-moment children increase their awareness of that bigger picture. I don't think you have a right or wrong answer here, as long as you are really, really honest with yourself and your family about both the short-and long-term drivers and impacts of this decision.
