What relation would your father’s only single sister’s sister-in-law be to you?

Rhonda will go see ballet but not the opera. Her favorite number is eight and she doesn’t like nine. She likes salmon but not trout. She hates Mondays and likes Wednesdays. Does she use a comb or a brush?

A family photo has the following:
1 grandfather, 1 grandmother
2 fathers, 2 mothers
6 children, 4 grandchildren
2 brothers, 2 sisters
3 sons, 3 daughters
1 father-in-law, 1 mother-in-law
1 daughter-in-law

It could show 29 different people, but what is the smallest number of people if could have?