Thanks for the feedback all.
I was unaware, and I think they are unaware, of the property tax consequences of buying out half from the other sibling. That might change things.
They are retired and I think pension + SS covers their current living expenses, but I am not privy to their financial details.
They are thinking of relocating in a few years, but probably won't do that until their family obligations in their current city are finished (parent in a long term care place). The relocation targets are currently fluid, but SF was one of the possibilities with current friend networks there, but they haven't lived there for 40 years.
I'll give my opinion to sell. If they want to move back they can always buy (or rent) another place.