Sarah, that's pretty fundamental information which perhaps might have been better to have provided upfront. As always there are non financial objectives which are much more important!
Don't see a lot of options really, unless you sell to your brother/sister at a discount. Doubt a bank would lend here.
I would not be blinded completely by the apparent loss of 500k divided by 3 people, less capital gains could be around 100 grand each, maybe slightly more. So it would not be a disaster to sell, though it would feel that way if the tenant passed away soon of course.
Can you sit and wait or is the desire to move so strong that you want to do so? Who is the elderly tenant, what relationship, age, health, etc. Any alternatives? Are you/could it be deemed that the existing tenant is inconvenient, ills being wished, etc?
Depending on your brother and sisters own financial positions, would they be able (prepared) to make available the 80 grand if the ownership of the house was changed.
eg. 80K divided by 1 million = 8% of the house valuation with the tenant. Could they give you 40 each for example and you transfer 4-5% of your ownership to them? They get the upside in terms of a higher price in the future, and you get your money. Or could one do for 8-10% and the 80 grand? Don't think you could use the higher valuation as they are taking risks, though may be useful to build in some conditions that if the tenant passed away in a very short period of time that you get some of the upside. (eg a notional interest rate cap is built into the money they give you now).
Though could get messy with siblings, lots can go wrong, emotions, jealousy potentially down the line, etc. You would all have to agree to terms and conditions, etc. Wouldn't write it off depending on the various financial positions, but could be bothersome all the same. Out of the box thinking required.