I'm a little stuck with this quest, thus far I've gone to the old hut on the cliff, examined the graves and solved the totem puzzle to get the note from the secret room. (I also had Dethmol have a look at it.) Afterwhich the final lines in the journal talk about how the fourth graves name is a mystery and then it leaves me with two markers, one on the beach and one in the house where there are no possible interactions available.

Do I simply need to explore more in the hope of finding another NPC to give another clue, or have I found a bug of some sort?

Have you talked with the sisters
the plot point on the beach is where they turn up but only at 23:00 best just to mediate there

hope this is helpful