First question from your post is, "What the heck did you do with your Time during the one whole year you were caretaking your relative?"

You have (presumably) paid the taxes and allowed your relatives as your guests to be on the property for purposes you agreed are voluntary and can be revoked at any time.

Your relatives probably don't mind because they don't pay state income tax. 

Invariably your related experience will be adequate and your good outside income will definitely help as well.I am not too far away from you and I have financed car washes previously.

What is your relation to the tenant?

Under those conditions, and based on your relative level of experience, you probably need an attorney.

What happens if the lease you type up for your relatives property doesn’t hold up in court?

Just because no income in 2017 doesn't mean you just get to choose to not reported your related expenses. 

Depending on your relation with the bank, they may lend you 50-70% of the loan balance.

If you go that route you need several documents to protect you and effectuate taking over mortgage payments (POA, notice to release, etc. )If you only have a strict 90 days to do this then in your agreement that should be spelled out but several things depend on your relative and how secure he wants to be in the fact that you will basically hand the house back over to him at the end of 90 days if you haven't sold it.