[Later] I did a little googling, and it does look like there's a lot of room there. They must have bought it when Joe Kennedy had lots of money and the rest of the country didn't. But still, there are a lot of Kennedy grandchildren.
And who's to say that her kids won't want to be buried with her, or what the world will look like decades from now?
FWIW, Ted and Jackie were buried at Arlington with Jack and Bobby. Presumably, Ethel could be buried there, apart from her own children.
Families buried together harkens back to a time when basically no one left their home town, except pioneers. Generation after generation married within a very small geographic area, usually within a limited number of other inter-related families. That situation no longer prevails.
I thought spaces at Arlington were reserved for heroes.
What the hell is Robert Kennedy doing taking up space for exactly three months’ service on a Navy ship after the war was over?