Brain Teasers # 4 - Family Photo

A family photo contained:

one grandfather, one grandmother,
two fathers, two mothers,
six children, four grandchildren,
two brothers, two sisters,
three sons, three daughters,
one father-in-law, one mother-in-law, one daughter-in-law.

29 people you may think, but no! What is the fewest number of people that could have been in the photo?

Please post your answer if you can solve this. The answer to this puzzle will be announced tomorrow.

In the meanwhile, here is the solution to yesterday's puzzle:

Take the she-goat to the other side. Go back, take cabbage, unload it on the other side where you load the she-goat, go back and unload it. Take the wolf to the other side where you unload it. Go back for the she-goat. That's it. 

Cheers!