Should I require an upfront fee, a contract, etc.

I also put lease break fees into my lease for situations like this but if the sisters are willing to cover the whole month, you may luck out and not lose any money or have to go to court at all. 

@Wayne BraultI know some brokers offer a flat fee listing on the MLS.

However if I factor in property management cost and gardener fee, I am in the hole closer to $300 a month.

In North Carolina, it's customary to have a "due diligence" fee when executing a purchase agreement.

Be sure to use the OK language for when rent is late, late fees and other rules that are often dictated by state law.  

Sure, we are out an option fee that way, but that's less than a major foundation issue, for example. 

Our option fee is $3900 to $10,000 depending on the price point of the property.

Obviously speak to an attorney, but document all fees for cleaning, and take from the security deposit.  

Check out the website real estate express, you can probably get your license for under $200 and then link up with a cloud broker and pay a $500 flat fee on every deal you close.