Bumping this thread.
Going to share all feats and info I got on the Brothers:
Before the Retcon, in the Crossovers, they were the Embodiments of the Marvel and DC Universes. I.e, their fictional continuities:

As this is incredibly hard to quantify, however, the best we can do is scale then upwards from The Living Tribunal. Giving them a rating of 1-B. However, they eventually were retconned in the comic
The Adventures of The X-Men #12.
The comic takes place in the previous iteration of the Universe, showing the why of it's demise. Namely, that a being called the Dweller in Darkness desires to destroy it, to consume the grief and despair of all beings who will perish with it. Thanks to the actions of that Universe's X-Men, a telepathic message of hope is send to all sentient life before the end, and all die without fear nor sadness, giving the Dweller nothing to feed on.

During the storyline, the Dweller in Darkness sees The Living Tribunal creating something: Namely, The Brothers.

Some people often focus on the "Energy that could vaporize galaxies" line, but they ignore what follows, and which is equally as important:
"Energy that could vaporize galaxies flares on his massive hand as two figures play out the endless conflict of existence... Two incalculably powerful siblings whom the Tribunal has a place for in his great scheme of things. But not just yet. First there must be the transdimensional consultation with his hooded, spectral ally"
The mentions of the Brothers fighting the "Endless conflict of existence", and the clear reference to the Spectre implies that it's not only the Brothers that are being retconned, but the entire Crossover. Rather than the two being infinitely-powerful embodiments of the Marvel and DC Continuities, they are single realities crafted by TLT.
Finally, after the previous Universe is destroyed, we get this page:
"A hand soon opening two allow Two Brothers too assume their roles as architects of new realities. And only this ageless judge of all things will know why."
This explicitly implies that the Two Brothers are entire universes, and that possibly the Marvel Brother is the embodiment of Earth 616, as directly after this scene we are shown the start of X-Men #1, redrawn, and a narration describing that what had been gone is born once more.

As such, I think that Post-Retcon Brothers should be Low 2-C.