Rahul's father is the only son of Rohan's Father. How is Rohan's wife related to Rahul?

Mother

Before answering this question we must be aware of the method to solve the blood relation questions.

For example : 

It is now clear, how to make charts in Blood relations. Now, shifting to another type, in this we solve the question in the following manner.

The chart for the above question is as follows: