Thanksgiving Riddle

I once again have to apoligize for the lack of blogging. When the school year comes around my "bloggable" experiences diminish along with my alotted blogging time. As of right now i have no good story, spiritual illustration or experience worth blogging about...so...I have a riddle for you to keep you busy until I get going on the blogging. If you think you know the answer, just let me know through a comment. We could have a family competition to see which one of you is really the smartest and deserves to be in grandma's will (smile). I will reveal the answer soon...HAVE FUN! (Hint: it shouldn't be too hard because I got the correct answer! You might want to scratch it out on a piece of paper though.)

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.

There’ll 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?