Listening to Évelyne Bloch-Dano talking about the Proust family, I noticed that she referred to Proust as an "older brother" and then excused and corrected herself: "elder brother." And there was I, a native speaker, wondering: What's the diff?
I checked my "Fowler's," H.W. Fowler's A Dictionary of Modern Usage (ed. Sir Ernest Gowers, 2nd ed., 1965), and found this compact explanation:
elder, -est. These forms are now almost confined to the indication of mere seniority among the members of a family; for this purpose the old- forms are not used except when the age has other than a comparative importance or when comparison is not the obvious point. Thus we say I have an elder (not older) brother in the simple sense a brother older than myself; but I have an older brother is possible in the sense a brother older than the one you know of; and Is there no older son? means Is there none more competent by age than this one? Outside this restricted use of family seniority, elder and eldest linger in a few contexts such as elders meaning persons whose age is supposed to demand the respect of the young, and as the titles of lay officers of the Presbyterian Church, the elder brethren of Trinity House, the elder hand at piquet, and elder statesman.A check of Google Book Search reveals that the distinction between eld- and old- is preserved in the New Fowler's Modern English Usage (ed. R. W. Burchfield, 3d. edition, 1996).
Elder, older, eldest, oldest, let's call the whole thing off.