Rätsel – Thanksgiving Dinner

Susan has invited her family to Thanksgiving Dinner. As her plates are not the best, Susan wants to buy new plates – one for each person at her Thanksgiving Dinner.

Therell be a grandfather, a 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 a daughter-in-law.

At least how many plates does Susan need to buy?

Note: Some members of her family are listed more than once (after all a father is also a son etc.).