If Callieach is one of the original forms of Mother Winter (which is pretty bloody obvious - literally she is known in Scotland as Beira, Queen of Winter), her counterpart is normally Brighide or Brigit.
Here is where it gets a little tricky. Brigit is the daughter of the Dagda (head god (Zeus-level) of the Tuatha De Dannan - the celtic gods) and married Bres (a Formorian). She is also normally associated with Minerva/Athena.
Somehow - back in ye olde times, I believe that when the Gods were putting all their punch in the fae, Athena was a big part of it. Along with Hecate, I think they created some sort of mega-deity that became the Queens. And they had a reason to split the Courts - it was necessary but I am not sure why. And somehow, I think Black Annis may have been Mab or maybe not. The whole scene in Ghost Story or Cold Days where she resembles a praying mantis and a cat struck me.
And yes, I sound nuts and have indeed lost control of this theory. Still though, anyone want to join in?