what are everyone's thoughts re: one 50+ sibling living in the parents' home (rent/bill free) - the other one independent - does the one 'residing' in the family home (not looking after parents or anything like that - quite the opposite actually :-( ) automatically become sitting tenant...
Ooh, that can be a minefield. I think a lot of it depends on the dynamic as to who owns/pays the rent/mortgage and the upkeep of the house. There's a big difference depending on whether it's effectively a grown-up child who never left home or the same grown-up child whose parent(s) live with them - particularly after one parent has died and it turns out more to be a co-dependent house-sharing dynamic with the surviving one rather than continuing the normal child-living-with-parents scenario into much later life.
I know of two people who didn't leave their family homes until late in life (in fact, one is now in his later years and still lives there, his parents long gone), but both assumed responsibility for the mortgages, bills and upkeep from their ageing widowed parents. That meant they had a head start on the property ladder but also provided free housing for their parents until old age/death, which seemed to work well for them and appears a fair deal to me.
However, in the scenario that you give, where you still live by choice as a 'child' in your 40s, 50s or older - maybe pay a token amount of board to your parents but you have your room in their house and your parents still do all of your cooking and washing for you, I think that's very different.
If you're an only child, no problem; but if you have siblings, I think you should either be prepared to sell and move out, buy them out or, if they're happy for it, pay a fair market rent for the half/two thirds/three quarters of their house that you're occupying.
None of this "But it's my home where I've always lived" as if you were still a dependent child and it never crossed your mind that this situation was very likely to happen when your elderly parents died. Even a rent-free life interest in the house would seem unfair, unless say, you had a serious disability which meant you'd never been able to move out; but even then, once your parents are dead and you're on your own, how is it any easier to live without them (with carers if necessary) in the house that had been theirs than in any other house?
Maybe if you had severe learning difficulties which meant you'd never been able to work, but could somebody with a mental age of a toddler really live alone in any house anyway?
I certainly don't agree with the idea that, because you've had a very easy ride for so long, that entitles you to continue it forever and those who made an effort to take responsibility for their own adult lives are penalised for it.