A questioner asks about the allowance of shaking the hand of his father’s mother-in-law from his second wife.
Answer: Your father’s mother-in-law (from his second wife) is not your mahram. Rather she is mahram to your father because she is the mother of his wife. So she is not the mahram of his son. Only the father’s wife is a mahram to his son, meaning, your stepmother is a mahram for you. As for your father’s mother-in-law (from his second wife), then she is a mahram for him. Likewise, your grandfather’s mother-in-law (from his second wife), then she is a mahram for your grandfather―but she is not a mahram for you because she is a non-relative to you.
Source: Fatāwa Nūr ‘alad-Darb of Shaikh Ibn Bāz, vol. 31, p. 311 (no. 134), Q. 5, Audio no. 239.
 Meaning, she is not like your actual grandmother.
 Meaning, son from his from his first wife.