I can see why you want to go, and understand the reasons. But you need to be very clear with yourselves, firstly, and then the wider family.
Your DS - does he know of your thoughts at all yet? Does he live near you at the moment, and do you see a lot of each other, help out, or just hang out? Or is it all phonecalls and short, less regular visits? If the former, that's a huge change, whereas the latter is far less so. And seperately, if you go abroad and they can't stay with you (why not have short visits if you have a spare room, you can put a sofa bed in living room and all paraphernalia goes into bedroom by day?), are there realistic options for their family to visit frequently - are there hotels or apartments nearby, are they suitable for families, are they cheap?
Your BIL - that's probably a harder nut to crack. At the moment, what does your "care" of MIL involve? Is it physical care, or more keeping her company, or less actual physical bodily care but things like getting shopping in or cooking meals she can reheat easily? Is she capable of staying at home now, and will the support she has continue as needed? If you are not there, what would BIL be likely to do - will he visit, will he organise practical things like shopping, will he watch over medical issues, will he pay for any aditional help needed? Do you all live in reasonably close proximity, or does he live more distant to MIL than you and DH? If you sit down and talk to BIL in terms of staying there, would he be amenable to you continuing to do a certain amount of practical things as you have the time, but contributing financially towards things that will make that job easier (like arranging an internet shop to be delivered while you are there - so you can keep MIL company while doing a batch cooking for her freezer using ingredients BIL has bought; or arranging for a taxi service account for her so that she can go out to get her hair done or could join a suitable club or group she might be interested in or whatever and maintain some independence by not needing a chauffeur i.e. you and your DH)?
TBH, while not feeling as harsh about it as some other posters, the reasoning you have put down here sounds selfish. Understandable in terms of finances, but with a thread of selfishness running through it.
I think you need to be very clear between you and DH about it all, and definitely let others know well in advance of actual moves.