The Kennedy's suck.
What I've read is that she was sneaking out of her boarding school at night (along with other non- or less-retarded girls). Lobotomies pretty reliably eliminate the will/motivation to do much of anything. They figured she'd be more "manageable" if she had a lobotomy. Being "pious" Catholics, they refused to go for the obvious solution: have her sterilized, and don't worry about it if she occasionally slips away and manages to have S-E-X.