Based on her father's will, Mary Sisson was alive and unmarried in June 1734, which seems to prove she was not the same woman as the one who was married to John Pearce, or who died in 1726. Her married sisters were named with their married names in his will, so I doubt he would have called her by her maiden name if she was married. There is also mention in his will of a deceased daughter, also married, so he did make provisions for surviving grandchildren, too. I think you'll have to look elsewhere for another 'candidate' for your ancestor's line. Sorry!