Would you sell your house & downsize to help your sibling?

I think you may be considering a plan which both parties may live to regret and might even affect your future relationship. Also, since you say that your sibling has no home and no income, this is something which does require some urgent action. Grannyben's suggestion then seems an excellent one and should stop you worrying. Without a home or job/s, why should it matter where your sibling is urgently housed, unless there are school children involved perhaps? You are obviously well placed to help, but you should not risk putting yourself in a worse position by selling up just because the comfort and security of someone you care for has taken a downward turn. You say that the current situation "will change in time". Then why not give it some time, suggest a move to live with you whilst all options are considered, the pressure is then off and much better solutions can be considered. It's possible that another job, and some accommodation in whatever part of the country is decided may be easier to come by than you think. Meanwhile you will be giving your sibling some breathing space, but not throwing your own life into disarray or saying goodbye to your pension. If this was your child, I could more easily understand your thinking in this way and wanting to offer a leg up, but even though your sibling might need some help at the moment, he/she is probably well able to make life decisions in normal circumstances - and without your involvement. Don't stress about it : if your offer of accommodation is unwelcome, then housing will have to be sought through the Local Authority as has been suggested.

Do not do this, keep your own security, if your family have no place of their own it’s up to social services to sort out accommodation.
Whatever has gone wrong it is serious and money may well make it worse.

You are a better sibling than I ever would be !
However, Charity begins at home. First, make sure you are financially secure. Dont give up the roof over your head.
If your sibling is in the UK, she/he will be entitled to Incone Support, Housing Benefit etc. Certainly help her in the short term, such as deposit for rental accommodation. Landlords do expect a substantial deposit .
Good luck to your sibling and yourself ...

Absolutely definitely not. I am afraid you have to think about your future.

I would think long and hard before committing to this course of action MarytheBookeeper are you sure all other avenues have been explored and that you cannot just help out in the short term. It's a big step to take and I'm not sure I could do it.
It's greatly to your credit that you are even thinking of this,

I was within 48 hours of being homeless and, like your sibling, I bad done nothing wrong whatsoever.
Thankfully, the local authority stepped in and house me. Oh, it was such a rough area and under any other circumstances I would have been appalled. It turned out the neighbours were absolutely lovely to me and I absolutely loved living there.
Could your sibling contact their local authority?

No.

I hurt for them.

Could you bear to see your sibling in agony & having a terrible time without a home to call their own?

We live at opposite ends of the country.

I wouldn’t if it’s going to put me in a lesser position.

I did.

Could your sibling come and live with you until they get on their feet?

Hmmm..
Honestly? I don't think I would.

I would worry about all the "what ifs".