Breaking down the relationship description: "She is the sister of my mother's husband's only daughter-in-law"
Starting from Rahul's perspective:
"My mother's husband" = Rahul's father
"My mother's husband's only daughter-in-law" = Rahul's father's only daughter-in-law
A daughter-in-law is the wife of one's son.
Therefore: Rahul's father's daughter-in-law = wife of Rahul's father's son
Since it specifies "only daughter-in-law", Rahul's father has exactly one daughter-in-law.
This means Rahul's father has a son who is married.
Rahul is the son of his father, so the only daughter-in-law refers to Rahul's wife.
"She is the sister of [Rahul's wife]"
The girl is the sister of Rahul's wife.
Therefore, the girl is Rahul's sister-in-law.
The answer is Option 3: Sister-in-law