The Kennedys are still powerful ? They had lost its seat in Massachusetts and all the men are dead and remains only the female Kennedys sons its true ?
You don't have to hold a seat to be powerful in American politics, like all politics it's also about money, respect and influence. The Kennedys have all three.
It's just not the "Kennedys" Kennedys, they have extended family through marriages like the Schrivers.