1) Is the brother of one’s grandfather / grandmother mehram for a girl and similarly is the sister of grandfather/grandmother mehram for a boy?

2) Are the aunts/uncles of spouse mehram?

3) To what extent is a father/mother in law mehram?

4) If the people mentioned in 1st question mehram then are they mehram for the spouse of that person also?

1- Yes, the brother of her grandfather is mahram to her, and the sister of his grandmother is mahram to him. He would be the girl’s great uncle, and she would be the boy’s great aunt.

2- No they are not mahram.

3- They are fully mahram. One can see them without hijab

4- No they are not mahram to the spouse.