My Nanna's will has been probated between my brother, my sister and myself. The amount of 95K to be divided evenly between us. My mother was an executor to the will but has since passed away. My sister is an executor to my mothers. She (my sister) is threatening that I will not get my share of the money unless I follow contingencies that she plans to set in place, such as attending a rehab. Is it legally possible for her to add contingencies to my Nanna's will?

None that I know of!
You say 'has been probated.' That means finished, past tense, and you are due your share forthwith.
Your mother died not long after your grandmother died? Did your mother have a will too?
(There's no confusion here about Nana's inheritance going to your mother first?)