Linux

In reality all those older relatives are rather easy. They don't have a clue what they use anyway and they usually are also using their devices so little that they are not conditioned to expect all the (often questionable) Windows design decisions either.

So there is really no reason to overthink it. Install whatever Linux distribution you are most used to operating in case they actually need support. Most of the time they won't because unlike Windows it does not just break randomly or simply slows down to a crawl with accumulated bloat over time.