I wonder who Hamlet thinks is going in that grave before he finally hears Laertes call her “my sister.” Like, sure, at first, it might be logical that it’s Polonius that has brought Laertes to the cemetery – but very quickly, there are many references to “her” and “she” and such – somehow Hamlet must be doing quite a lot of mental gymnastics to be surprised when Laertes says “my sister.”
I mean – if it were a line like, “Not, the fair Ophelia!” then it would be a truth that had perhaps been dawning on him slowly but he is somehow caught completely by surprise.
And it’s not as if he is uncurious about who the dead person is. He’s first asked the gravedigger who is to be buried there. Then he puzzles out that the dead person must be of some estate due to the accouterments and the company. But it takes Laertes saying “my sister” before he gets it. Does he think Laertes has an aunt or something?