The girl was called "retarded." From what I read, she may have been a bit slow. She was by no means as ruthless as the rest of the Kennedy spawn. If anything, she was said to be "promiscuous." Apparently, that trait was only acceptable when demonstrated by Kennedy males.
She (I believe her name was Kathleen Kennedy) was reportedly mentally retarded, and it was in her honor that Eunice Shriver started the Special Olympics. The zombie reference, however, was to Kathleen’s state after her father (again, reportedly) had her lobotomized.