Quote from @Seth Kristian: My In-laws had found a rental last year...It sounds like a lot of these problems should have been obvious from the beginning, so your relatives are at fault for renting it without investigating more closely.

The absence of a rent increase for 6 years, frankly, is something that your relative should have observed based on the cash flow he had been receiving from the property over that lengthy period of time. 

You can try to arbitrate it as @Nathan G. suggests to help out your relative, or just help her out with payment yourself and let Joe know he's going to hit the bricks when his lease is up if he doesn't pony up some cash for an honest mistake.

You're going to be paying tax on 2m instead of your relatively low locked in property tax on your original purchase(i'm assuming its around 1m) I think planning for family takes more nuance than just numbers.

Did you not say that another tenant uses that ame space along with your relative?

What is your relation to the property?

Now if you don't want to lose the deposit you can try not to by purchasing but your relator and lawyer sound like they aren't going to help you and you don't think its a good deal with lead remediation so your best bet may be to walk  (just moved out of Shrewsbury). 

Would be a good idea to consult with a title attorney to determine how you and your relative should hold title on this home, given these special circumstances. 

I'm out here in San Antonio, are your relatives looking for a place to rent or buy?

Unless your related to them or a friend it’s only a $10,000 sales deal.