I am so sorry for your loss and for the problems that have ensued.  Here is the problem that generally crops up with real estate matters: interests or transactions in real property have to be in writing in order to be valid.  Other wise it violates what is known as the statute of frauds.  The fact that the one sister acknowledges only one third ownership in the property may not be enough.  How exactly the Will is worded may be a plus.  I would take a copy of the Will to an attorney to review.  Then for proof you would need copies of payments made for mortgages, taxes, improvements, etc.  Good luck.