Greatly appreciate the feedback.  I further inquired just to make sure my understanding of facts of the situation is correct as sometime having a written narrative provides more clarity that verbal.  Update to the scenario:

There are three siblings.  Only one parent is included in transaction as follows: 

One of the sibling and mother purchased property back in 1980 (50/50% interest each).  Years later due to health concerns, mother 'gave'*** her 50% share to each of the three children (includes a portion to the initial sibling purchasor).  The sibling that initially purchased the property with mom is not my client.  The portion received by my client would be 1/3 of the 50% that was transferred from mother.  

Mother passed away years ago and the three children continued their ownership interest in the property until now and have decided to sell.  Gain would be 1/6th of net gain measured against date of death basis? 

***I will clarify on the term 'gave'.  Initially it was mother sold interest at a very small amount to children.