The husband's grandchildren are mahram (forbidden relatives) to the maternal grandmother

The questioner:
May Allah reward you with good. A sister of my father, who has male children, asks: Are these children mahram to my mother, or are they strangers to her? I have a sister from my father, who has male children; are these children mahram to my mother, or are they strangers to her?

The Shaykh:
You mean she has brothers from a different mother.

The questioner:
She has a sister from her father, from a different mother, yes, and this sister has male children, and at that point she asks whether they are strangers to her mother, or whether they are among the mahram?

The Shaykh:
Yes, these women are forbidden to her [the mother]; because she is the wife of their grandfather, because she is the wife of their grandfather on their mother's side, so she falls under the saying of Allah the Exalted: The father includes the father, and the grandfather from the father's side or from the mother's side, all of them are called fathers, and their wives are forbidden to their sons, and to their grandsons, and to their daughters' sons, yes.