Now draw the Family tree diagram using the following notations:

From the given information:
1. Kartik is wife of Nitish. Nitish is the grandfather of Resav. Kartik has only one child(son) who is married to Tousif’s child. Tousif has only two children,i.e. one son, and one daughter.
From these statements, we can clearly determine that Nitish is the husband of Kartik and they have only one male child who is married to the daughter of Tousif who has only two children, i.e. one son, and one daughter. Since Nitish has only one child(son), Resav is the son or daughter of the only son of Nitish.

2. Krishna is the granddaughter of Tousif. Ranjit is married to the son of Tousif. Krishna is Ranjit’s child.
From these statements, it can be clearly determined that Ranjit who is married to the son of Tousif has a daughter whose name is Krishna.

3. Varun is the brother-in-law of the son of Tousif.
From this statement, it can be said that Varun is the husband of the daughter of Tousif.

4. Krishna is the niece of Sahil. Sahil and Rana are the children of Tousif.
From these statements, it is clear that Sahil is the sister of Ranjit’s husband. So, Sahil is the daughter of Tousif. Therefore, Rana is the son of Tousif as Tousif has only two children,i.e. one son and one daughter.

We cannot determine the gender of Resav and Tousif.
So the final family tree diagram is:

From the above family tree diagram, it can be stated that Resav is the son or daughter of Sahil.
Sahil is the daughter of Tousif.
So, Resav is either the grandson or the granddaughter of Tousif.
Hence, We cannot determine how Resav is related to Tousif since we do not know the gender of Resav.