I have a problem

[Miles White]

I have recently found a great property to put under contract (excellent), the owner is very motivated in getting the property off her hands, she has a buyer in mind who wants the property(her sister)  and even agreed to lower the offer I originally gave her (sounds like an easy sale right). Well heres is when things get tricky, she already has a realtor working to sell the property (first problem), and her sister is on the title which would make it very difficult to assign her the home (second problem). I want to finally get my first Whole sale deal completed but I need some help figuring out how.

Here is my question is there anyway I can still assign the property to the sister and get this deal completed or should I just look for  another cash buyer?