Could it be the Weirdstone of Brisingamen by Alan Garner. It is not explicitly Aurthurian but it has a character identified as the Morrigan and a wizard called Cadellin Silverbrow that I always think of as more or less equating to Merlin. The main charcters are a brother and sister whose parents are overseas. It somewhat matches your other details and there is a sequel. I'm not sure if you'd call it obscure, it depends where you are I suppose.

https://en.wikipedia.org/wiki/The_Weirdstone_of_Brisingamen

Could it be The Weirdstone of Brisingamen by Alan Garner?

It is not explicitly Arthurian but it has a character identified as the Morrigan and a wizard called Cadellin Silverbrow that I always think of as more or less equating to Merlin. The main charcters are a brother and sister whose parents are overseas. It somewhat matches your other details and there is a sequel. I'm not sure if you'd call it obscure, it depends where you are I suppose.