The grandfather’s wife is a mahram
Question

My maternal grandfather has two wives. Is it permissible for me to see his second wife, who is not my mother’s mother?
Praise be to Allah.The grandfather’s wife is a mahram, so it is permissible for you to see her, shake hands with her, be alone with her, and travel with her.
Shaykh Ibn Baaz (may Allah have mercy on him) said:
The grandfather’s wife is a mahram, whether the grandfather is on the mother’s side or on the father’s side, because Allah, may He be glorified, says (interpretation of the meaning):
{And do not marry those [women] whom your fathers married} [an-Nisa’ 4:22].
This includes fathers and grandfathers on both sides, on the mother’s side and on the father’s side. Therefore she is a mahram to him [the grandson], and he may travel with her and be alone with her; there is nothing wrong with that.
End quote from the website of Shaykh Ibn Baaz.
And Allah knows best.