Step 1: Read the key sentence carefully.
We are told "Mira's mother-in-law's mother is my grandmother". Let us anchor everything to the speaker ("my").
Step 2: Place the grandmother.
The speaker's grandmother is also Mira's mother-in-law's mother. So Mira's mother-in-law and one of the speaker's parents share the same mother, i.e. that grandmother.
Step 3: Find how Mira's mother-in-law connects to the speaker.
Since Mira's mother-in-law is a daughter of the speaker's grandmother, she belongs to the same generation as the speaker's parent. Hence Mira's mother-in-law is the speaker's aunt (the speaker's parent's sister).
Step 4: Identify Mira's husband.
Mira's mother-in-law is the mother of Mira's husband. So Mira's husband is the son of the speaker's aunt, which makes Mira's husband the speaker's cousin (aunt's son).
Step 5: Use the unmarried clue to rule out the wrong branch.
"All my mother's offsprings are unmarried" tells us the speaker and the speaker's own siblings are not married. So Mira cannot be a brother's wife of the speaker, because no sibling is married. This eliminates option 4.
Step 6: State the relationship.
Mira is married to the speaker's aunt's son, so Mira is the wife of the speaker's aunt's son.
\[ \boxed{\text{Mira is the wife of my aunt's son}} \]