This may have been kicked around before, but I haven't been able to find a similar theory.  My apologies if so.

We meet the Mothers for the second time in Cold Days, but in a rather unique way.  Harry attempts to summon Mother Winter... and it works.  His summoning reveals, I think, a very important gap in our knowledge.

Premise:
The Mothers, along with a heretofore unknown entity, are the Fates.

Background:
The Fates are three women that appear in various mythologies who control the destinies of mortals.  I will use the Greek names for concreteness.  There is Clotho, the spinner of the thread of life, Lachesis, who measured the length, and Atropos, who cut it with her shears.

Another rather prevalent mythos surrounding thee women goddesses is the idea of the Mother, the Maiden, and the Crone.  One can map the three Fates onto the three goddesses easily.

In most of these mythos, the Fates are absurdly powerful, up to deciding on the fates of the Gods themselves.

Claim:
Mother Winter is the Atropos, the Crone; Mother Summer is Lachesis, the Mother; we have yet to identify the Maiden, Clotho.

Supporting evidence:
Perhaps the most obvious evidence is that Harry attempts to summon Mother Winter using the Greek and Norse names for Atropos.

“I am Harry Dresden, the Winter Knight, and I needs must speak with thee! Athropos! Skuld! Mother Winter, I summon thee!

Mother Winter’s rocker creaked, though it didn’t really seem to move. “He knew certain names. He was not wholly stupid in choosing them, or wholly wrong in using them.”

While on the subject of names, it is implied twice that there is a name that references both Mother Summer and Winter together.  I suggest that it would be the name of the Fates, which is Moirai in Greek.

Mother Summer’s bright green eyes narrowed. “Did he . . . ?”
“No,” croaked Mother Winter. “Not that one."

Mother Summer’s smile appeared for a moment, dazzling me, and then was gone again. “It was not an imbecilic guess,” she said. “And, yes, she has been known by such names before. But you’ve only guessed the name of one of her masks—not our most powerful name.”

Additional evidence is provided by looking at the personalities of Mother Summer and Mother Winter.  Mother Winter is old (so old she can barely walk), does not suffer fools, and is generally not the nicest Fae around.  She is perpetually shrouded in her cloak and has an absolutely evil cackle.  She also has the requisite shears:

Mother Winter reached down with her withered hands, and took up a pair of rusted shears. She cut the trailing threads and passed me the cloth.

Mother Summer, on the other hand, is much more matronly and kind.  She bustles around their cottage and tends the house.  She actually tries to put Harry at ease and is generally pleasant.

Speculation:
If we accept that Mothers Winter and Summer have a corresponding Maiden, then we must speculate on who that would be.  My best guess at the moment is that the role of the Maiden is played by the Summer Lady.  It is not known what true purpose the Ladies' power fulfills, or at least it hasn't been revealed.

“And her duties?” I asked. “What is the purpose of the Winter Lady?”
“That is for her to know,” Mab said.

Given that Clotho is associated with life and giving birth, it seems more reasonable that the Summer Lady would serve in this role rather than the Winter Lady.  It would also fit with the "youngness" of the Summer Lady, and it is not unreasonable to draw the connection between the Lady and Mother.

Conclusion:
I think that Mother Summer and Winter are two of the three Fates, which necessitates identifying the third Fate, the Maiden.