There are six persons 1, 2, 3, 4, 5 and 6. 3 is the sister of 6. 2 is the brother of 5's husband. 4 is the father of 1 and grandfather of 6. There are two fathers, three brothers and a mother in the group. Which of the following is a group of brothers ?

4 is father of 1 and grandfather of 6. So, 1 is father of 6. Thus, 4 and 1 are the two fathers.
3 is the sister of 6. So, 3 is the daughter of 1. Since there is only one mother, it is evident that 5 is the wife of 1 and hence the mother of  3 and 6. So, 2 is brother of 1. There are three brothers. So, 6 is the brother of 3. Hence, 2 and 1 are brothers. 6 is the brother of 3. So, three brothers are 1, 2, 6.