Part of the mind of Ilúvatar

[Ibrîniðilpathânezel]

If Melkor is the mightiest in the thought of Eru, how can Manwë be called coëval with him?

Because coëval means that he is a contemporary of Melkor's, that he had his origins at or near the same time as Melkor. Melkor was "born" first, Manwë probably came second. I think that many people interpret coëval as meaning "equal," but that is not so, and I haven't found a definition in any of my available dictionaries that suggests equality of power, rank, or such in the term, merely a similarity of age or origin.

As for their being brothers, I have always viewed them as being opposite sides of the same coin -- light, dark; good, bad; heavens, earth; etc. They are different in temperament, and in that difference lies the link between them, in my mind. The two sides of a single coin can be utterly unalike, and yet be part of the same coin. Despite their differences, there is a bond between them, and in Eru's mind, this makes them brothers. I'm from a largish family, and most of my siblings and I are quite unalike, but that does not change the fact that we are siblings.

That's how I've seen it, anyway.