At a family reunion were the following people: one grandfather, one grandmother, two fathers, two mothers, four children, three grandchildren, one brother, two sisters, two sons, two daughters, one father-in-law, one mother-in-law, and one daughter-in-law. But not as many people attended as it sounds. How many person were there?

Since there are Grandparents(Represented by A&B)  and Grand Children there will be 3 level Family Tree. 

Since there is one Daughter-in-law, its possible when they have a son(C). Then Daughter in Law (D). Also they have 3 grandchildren(E,F and G). It can be represented in the family tree in the following manner

Furthermore, there are 2 sisters and 1 brother. It is possible when one of the grandchildren is Male.

The above family tree satisfies all the required conditions. Total 7 person were there