Would be simpler to “buy” it ie transfer ownership to you. Then it’s all up to you. Simpler too - we got a stone chip in the middle of the windscreen & the insurance sent a firm to our house to replace it: more explanations if you’re travelling under someone else’s insurance.
Otherwise you will have to be named on their insurance (we did that for friends who borrowed ours for a fortnight’s trip) & they will have to say where it’s normally being kept (they might keep it on a drive, you might keep it on the road). And you’d reimburse them for the tax - the V whatever form is posted to their address, not yours.
Surely the simplest way to do it is get added to your sisters insurance policy and if there's an extra cost you pay it.
And just give her the money for a years road tax.
------------- XVI yes?
As well is two words!
How does a sage know everything about everything? or does he? or does he just think he does?
Remember, if you buy something you bought it, not brought it.
Amazing family weekend with old steam engines, classic car displays, market stalls, and full catering and bar. And camping on site - Save £25 by booking in advance.