First thought: Mother Summer's more likely to be Clotho than Lachesis.
Why: Mother Winter is Atropos, the cutter of the threads, associated with death, destruction, and endings -- definitely fit. And Mother Summer should be the opposite of that -- associated with birth, creation, and beginnings. That's Clotho.
Lachesis, the middle Fate, is in many ways the most powerful and important. She controls everything else that goes on in between birth and death. Pretty much everything we think of as our fate or destiny.
I'd imagine that whoever wears the mantle of Lachesis is actually even more powerful than the two Mothers, at least by a little bit.
My WAG: The White God snagged the mantle of Lachesis (along with a *BUNCH* of others, like "Allfather", which Odin used to have) during his rise to power in the first millennium.
Another WAG: The triparte Hindu gods -- Brahma, Vishnu, and Shiva are mantles that are equivalent to the three Fate mantles, so Mother Winter would answer to Shiva as easily as Atropos.
Final, and wildest WAG: The Name that Mother Summer was worried Harry might have guessed? That other tripart god -- the Christian Trinity. Of course the three parts don't match up at all well with the Fates or the Hindu Gods... but there are three parts. This one falls apart, though, when you try to imagine that Mother Winter is the Holy Spirit or Mother Summer is Jesus Christ (or vice versa -- I can see arguments either way). It seems like a bit of a stretch, and the mythologies don't match up well.
The two of them together sharing a mantle like Gaia (Mother Nature, Mother Earth, whatever) seems more likely -- though I'm not sure we've ever seen more than one "person" sharing a mantle like that. Is it possible that the Mother is just one person, but at Her power level She can manifest separately in each of her mantles without worrying about the fact that she's in two places at once doing two different things?