Who was at the dinner?

One evening, in a house at the dinner came together:

a grandfather, a grandmother,
2 fathers, 2 mothers,
4 children,
3 grandchildren,
a brother,
2 sisters,
2 daughters,
a father-in-law, a mother-in-law,
and
a daughter-in-law.

But, in reality, there wasn't as many people as it seems at first.

How many persons were at the dinner, and who were they?