The History of Galadriel and Celeborn

[Galin]

I think Findegil is refering to a section of Morgoth's Ring dealing with the reborn fea, although that idea itself (as a mode of reincarnation among Elves) was ultimately rejected.

'For the marriages of the Eldar do not take place between close kin (...) By 'close kin' for this purpose was meant members of one 'house', especially sisters and brothers. None of the Eldar married those in a direct line of descent, nor children of the same parents, nor the sister or brother of either of their parents; nor did they wed 'half-sisters' or 'half-brothers' [these terms had a special meaning among the Eldar: Galin] (...) Otherwise 'first cousins', as we should say, might marry, but seldom did so, or desired to do so, unless one of the parents of each were far-sundered in kin.'

Morgoth's Ring, Laws A, The Later Quenta Silmarillion II

Of Maeglin would be a later work in any case -- or at least Tolkien worked on parts of it very late, but anyway I assume this must be the text Findegil is referring to.