All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His Slave and Messenger.
Allaah The Exalted named the Mahram women in the Quran and we have highlighted their categories in Fatwa 85868. The fact that a woman is the daughter of your grandfather's stepdaughter does not make her one of your Mahram women as she does not belong to any of the categories of Mahram women. Rather, she is lawful to you as stated in the verse that reads (what means): {And lawful to you are (all others) beyond these...} [Quran 4:24]
Therefore, she is not among the women you are forbidden to marry and you may marry her in the absence of any other religious impediment such as if she already has a husband or she is undergoing the ʻIddah (waiting period) or the like. The fact that she got married twice in the past is not a religious impediment to your marriage to her and does not make her one of your Mahram women.
Allaah Knows best.