I think that's just in the same arena as all the other nonsense Trump has to say; the Rafael/LHO "link", etc. No one really cares if it's true or not. I should say, not enough people really care if it's true or not. It's Trump's m.o.
It was a great smear, timed to come just before the Northeastern State primaries, in a region where the Kennedys are still looked upon as 'royalty' by many. Like the other smears which were trotted out one or two days before an election or caucus, targeted toward the demographics in the regions involved, and designed to have the maximum negative impact on his opponents, specifically Cruz.
I have little doubt the folks in his marketing department burned the midnight oil and earned yuuge bonuses for their efforts, whether they actually got paid a dime extra (on salary) or not.