Her sister is doing most of the hand holding for her but the problem is how binding is the lease agreement if she can not see it?

To be honest I need one rented before I can risk starting the other or else I might be in a financial bind.

They had a legal contract and obligation and broke it, leaving you in a big bind.  

Nice bluff, is my opinion.Because when they accepted money from you, they formed a binding contract.  

For all you know, she'll decide she wants another 18 days and another 18 days ad infinitum.So, just let her know you will not be extending the lease, and will be happy to accommodate her with an eviction on her record, and a nice fat judgment to go with it, if she wants to play nasty.You're going to be out money on this tenant no matter what, I'd bet.  

It has put us in a bit of a bind.

They tend to have assets and income with reserves that can last years or longer if needed.It's the medium to lower income areas that are highly populated that live hand to mouth that can really be in a bind.

I need to cut the fat.  

That's the bind that a lot of BP folks are in, they want to quit their day jobs rather than increase their equity. 

He is just now claiming a misunderstanding / misinterpretation of the deal.well if its in writing and legal binding contract then i dont think there is much to worry about..