@Jonathan Billing You have several different challenges and opportunities depending on your answers.
1. Is the house your F-I-L currently owns a rental property? Or is it his personal residence?
2. If he is doing a 1031 exchange into the home in WA and then rents it to you, he now has a continuing or new rental property. Depending on his age, if he is planning on willing the house to you, he can do a cost segregation study on the rental property for the difference between his relinquished property basis and the new one. In this situation, when he passes, there is no recapture for him (he could use that "free loan/cash-flow from the IRS" in whatever way he wants). You would inherit the property at the current market value. If that property on transfer to you is an investment rental property for you, you could do another cost segregation study to expedite your depreciation.
3. If you buy the property from him and he had previously done a 1031 exchange on it as a rental property, he would owe taxes upon sale. Unless, he did another 1031 exchange into a new rental property.
4. If your F-I-L did a rent-to-own, it is still a rental property for him and it is not a owned property until you have paid off the property or inherited it.
I am not a tax attorney or CPA. I am an expert in cost segregation and what happens when a property is sold, 1031 exchanges or inherited. If you need additional information, I can help or put you in touch with a good tax attorney and/or CPA. You need to know your options and what is best for your family's tax situation.