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 were there, and who were they?

simple

total 7 people
there are two old wife, husband (grandfather and father in law), (grandmother and mother in law)
one is their son (who is also father and one of four children)
one is his wife (also mother and daughter in law)
three are their children (one brother, two sisters)

hierarchy

grandfather-----------grandmother
|___________________|
..................|
.................father---------mother
...................|____________|____________________
.............................|....................|..........................|
.........................brother.............sis........................sis

thanku