If I had to guess, the landlord owned this house and your FIL wanted to buy it on a land contract, thus the agreement/contract they had was between the landlord and your FIL. There could have been a "verbal" agreement that FIL did not live there, but you was with your children, thus when you paid the "rent" to the landlord did not care, since all he cared about was the "monthly" rent.
So, just found out that the FIL died 2 years ago, but since paying rent may not have known, but after hearing this, realized that possibly in this contract that FIL and landlord had, may have had a "balloon" payment due, if he died, or something else, that would revert back to the landlord owning the house (free and clear) regardless how much was paid in the past.
Or, even if the deed showed both names on it, just like in a marriage will convert to the other person as sole ownership.
Odds are you can't get a $15,000 loan with short notice and he knows this.
But, the odd thing about this arrangement, is that you paid the property taxes, which a renter (never) pays, but the owner, which could be in your favor, if you can prove you in fact paid it vs FIL or the landlord.
Like the others stated, you MUST make an appointment in person with a lawyer for legal advice. Many will give you 1 short free consult. But the other advantage of a lawyer, is that this lawyer might be able to (continue) the 30 days to maybe 60 or 90 days, to give you time to get the $15,000 loan to pay this off.
good luck and please talk to a lawyer.