There are so many caveats and unknowns here that this is just an attempt at an idea...Option 1, the four grandchildren living in the property buy out the other two:-Estimate your PITI payments based on the expected payout to the 2 grandchildren being bought out.

I've never had any issue or problems with wholesaling or assignments or options or double closes, all of which I've done.  

Someone mentioned Quickbooks, also another good option.

The sellers don’t want to return the earnest money, creating more problems.If anyone has experience with this, could you advise on legal options?

Yes the interest rate is higher but it gives you optionality, in case your extra money for helping pay the 15 yr note disappears or your investment goals or priorities change for that money.

Another option if you both have self-directed IRAs is partnering them together. 

Other options?

I would highly consider house hacking if that's an option for you. 

Hi Brittany - @Samuel Eddinger provided you some great options for meet ups!

Thus, when the loan comes due, the property could easily be worth less than the loan amount, leaving the owner with very few options, none of them good.