Rosemary. She was JFK, RFK, and Ted's sister.

It was purportedly due to mental issues she had that may have been caused during birth because they had her mom keep her legs closed while she was in the birth canal for hours until a doctor got there. Whether she was truly challenged, depressed, or some combination we'll never know. But it's obvious the procedure was a disaster and rendered her barely functional for the rest of her life.