Is the grandfather's wife a mahram (a relative one may not marry)?

Question:
The sender says — or the female sender says — to our honorable Shaykh: I am raising this problem to your eminence, and its gist is that I married a man who was previously married, but his wife passed away, leaving him three grown daughters. They received their inheritance and married men from outside the village where we live, and they gave birth to sons and daughters. The sons have grown up, and they call me "Grandmother." I interact with them and ride with them in the car, whether to visit relatives, go to the hospital, or for other reasons. My question is: Is there any sin on me for interacting with them, and for riding with them in the car, even without a mahram? I ask your eminence for guidance, and I offer you my sincere thanks.

Answer:
There is no blame on you for that, and they are among your mahram (relatives one may not marry), because you are the wife of their grandfather, because you are the wife of their grandfather on their mother's side. Yes. And Allah, Glorified and Exalted is He, when He mentioned the mahram (relatives one may not marry), said: The term "your fathers" includes the father and the grandfathers, both on the mother's side and on the father's side. Yes.