Amit said - "This girl is the wife of the grandson of my mother". How is Amit…

Concept: In a family tree, a person's mother's grandson is a son of one of that mother's children. Read on its own, without any other relative mentioned, the simplest and intended reading is that this grandson is the speaker's own son -- since the speaker is themself one of the mother's children.

Applying this to the statement:

1. Amit's mother's grandson = Amit's son (by the concept above).

2. The girl is the wife of Amit's son, so the girl is Amit's daughter-in-law.

3. The father of one's daughter-in-law's husband (that is, the father of one's own son) is called the father-in-law of that daughter-in-law. So Amit is the girl's father-in-law.

Cross-check: Building the family tree downward from Amit's mother -- Amit's mother, then Amit (her son), then Amit's son, then Amit's son's wife (the girl). Counting up from the girl: her husband's father is her father-in-law, confirming Amit is the girl's father-in-law, and ruling out the other listed relations (brother, grandfather, husband), which would each require a different generational position not supported by the statement.