We find out that Ursula Monkton is Skarthach of the Keep or "fleas" as the Hempstocks call them, but I can't recall finding out what the Hempstocks are.
I thought they may have been witches, but this extract states otherwise:
'Rowanberry and red thread, stop a witch in her speed,' I recited. It was something I had read in a book. 'That'd work, and work well,' said Lettie, 'if there was any witches involved in all this. But there's not.'
Lettie of course could have been lying, but I don't think so. Does anyone have any idea what the family could be?