This is a fantastic leverage opportunity to get a price reduction or owner carry, as the alternative is admitting to the IRS that they lied.

You all have provided some creative alternatives that I will dive into and see what works with the seller and lender.

It seems like a reasonable alternative to a month to month lease, without it being a month to month lease.

Another alternative is to split it per tenant.

Alternatively, you could pay him a percentage on "top" of the work he supervises to get the house in shape.I would also think a "lease-up" fee would be appropriate. 

Your other alternative is to go completely electric.

But also give ample warning so tenants have time to find alternate housing .

An alternative will be to think like a renter, search roommates or craigslist by zip code and other criteria that matches your room. sort your results appropriately to find highest and lowest rents, average them out, that should be work as a magic number to attract prospects ceteris paribus.

The house is listed until the siblings offer some sort of alternative.

If this came across my desk from one of my clients I would have rejected it without a letter of clarification from both parties as to why this was necessary and why traditional alternatives were not acceptable.