what is the ruling on marrying the daughter of the stepson in case one is not married to her grandmother?
It is unlawful for you to marry any of the descendants of your wife with whom you have consummated marriage, whether her daughters, granddaughters or their descendants according to the generality of the saying of Allah (Exalted be He):
...your step daughters under your guardianship, born of your wives to whom you have gone in
If you have not consummated marriage with the grandmother, it is lawful for you to marry the stepson's daughter after separation with her grandmother whether by her death or divorce after the elapse of her waiting period. (Part No. 18; Page No. 224) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.