Read this in the early 90s but was probably from the 70s or 80s. The setting was Celtic or Arthurian. It involved transportation between the real world and the magical word. The characters were orphans and may have been siblings. They each had a magic item. Merlin and Morgan le Fey were featured. There were 2 or 3 books.
This may be the Magic Tree House Series.
- Read this in the early 90s but was probably from the 70s or 80s.
This series began being published in 1992
- The setting was Celtic or Arthurian.
- It involved transportation between the real world and the magical word.
The books dealt with magical portals and time travel - including, but not limited to Celtic/Arthurian times.
- The characters were orphans and may have been siblings.
The main characters are Jack and Annie. I cannot find any note regarding them being orphans, but they are siblings.
- They each had a magic item. Merlin and Morgan le Fey were featured.
I count four books (books 5-9 ) specifically dealing with Morgan Le Fey tasking them with finding magical objects. Merlin shows up in later books.
- There were 2 or 3 books.
Here is the biggest discrepancy. There were 52 books, but not all of them dealt with Morgan or Merlin.
Sounds quite like Susan Cooper's excellent The Dark Is Rising sequence. Most of the details match, especially for the fourth and fifth books, except that the protagonists weren't orphans or siblings.
There was an unsuccessful film of the second book.
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.