Usage of "elder" and "eldest" in degrees of comparison

word-usage comparatives superlative-degree

I think that's fine, but other people evidently disagree. I found discussion of the topic of whether "second eldest" is a correct turn of expression in a court case, actually:

It is said, "second eldest" is not grammar; there can only be one "eldest". I do not agree in that. I suppose that it would be good grammar to say "A, B and C, are the three oldest men in the parish."

(Thellusson v. Rendlesham [1858-59], in The English Reports, Volume XI: House of Lords, Containing House of Lords Cases (Clark's), Volumes 7 to 11)

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