Right at the end of the chapter The Disappearance of Jill - it's the final sentence. And I was wrong - it is attributed to a party, but he's only identified as "the oldest Dwarf".
I was just looking for it but couldn't find it) there may have been some kind of community that Jadis built up around her before she invaded Narnia and which continued her tradition in the North.
No, the Green Witch is no sibling of Jadis. Jadis left all her relatives lying dead on the planet Charn. And for Jadis to be able to come back AS the Green Witch would be denying the omnipotence of Aslan, saying that He could no more keep her dead than she could keep Him dead. The very LAST thing Mr. Lewis ever intended was to imagine a cosmic dualism in which the supreme evil power could be EQUAL to the supreme good power.
However, as both come from the North (whereabouts is that quote from PotW? I was just looking for it but couldn't find it) there may have been some kind of community that Jadis built up around her before she invaded Narnia and which continued her tradition in the North.
I would say she was somewhat related to the Hags, having put on a beautifying spell on herself. Or she may have been, like Coriakin, an exiled star on punishment. She was the only mortal shape-shifter we knew about in the Chronicles. Only Aslan ever assumed another shape other than his Lion. He was an Albatros and a Lamb (and possibly the White Stag, but that's another argument).
if a hag could beautify itself, wouldn't the one in PC have done so to make herself more acceptable to Caspian?