Party Puzzle

I want to invite a family to a party. The family has 1 grandfather, 1 grandmother, 2 fathers, 2 mothers, 4 children, 3 grandchildren, 1 brother, 2 sisters, 2 sons, 2 daughters, 1 father-in-law, 1 mother-in-law, and 1 daughter-in-law. A total of 23 people, you might think, but that's not correct. Oh, how many people are there?

Answer: