Solution

Mira's mother-in-law means the mother of Mira's husband.

So, Mira's mother-in-law's mother means the grandmother of Mira's husband.

It is given that Mira's mother-in-law's mother is my grandmother.

So, Mira's husband's mother can be my aunt.

That means Mira's husband is my aunt's son.

Also, all my mother's offsprings are unmarried, so Mira cannot be my brother's wife.

Therefore, Mira is the wife of my aunt's son.