My wife has a daughter from her ex-husband i.e. a step daughter, who got married and gave birth to a girl. Is the daughter of my step-daughter considered a Mahram (a spouse or an unmarriageable relative) to me ?
The daughter of your wife's daughter is a stepdaughter to you, for the general Saying of Allah (Exalted be He):
...your step daughters under your guardianship, born of your wives to whom you have gone in
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his Family, and Companions.