Translation:He is the least happy of the siblings.
I'm inclined here to say "from among" instead of "of" since it feels a bit more precise. And why would saying "his" siblings be wrong since we are obviously discussing the virtues & failings of a particular family?
Writing "his siblings" doesn't really include the "he"; you're referring to the rest of his brothers and sisters, excluding the subject of the sentence.
I'm guessing that you meant brothers & sisters. ;)
I'm not quite parsing what you are saying here. Let me see if I understand this; "his siblings" is incorrect since "the siblings" doesn't relate directly to the "he" in the subject? Do I have that correct? Because if I do it still doesn't make sense to me.
I'm not sure what you mean by that guess ;)
In any case, if you write "Fred and his siblings, Paul and Jenna", you're not including Fred ("he") among his siblings (Paul and Jenna), even though they're all siblings together (Fred, Paul and Jenna). That's why "He is the least happy of his siblings" doesn't sound right (because Fred is not included in the sibling group here).
To use the names you provided I will try rephrasing the sentence, in English, as I read the Esperanto above.
Fred is the least happy of the siblings; who are Fred, Paul Jenna.
Taking Fred out of the trio of siblings makes a very nonsensical statement. Saying: Fred is the least happy of the two/three/infinite siblings involved suddenly makes more sense, but the statement is made without suggestion of numbers except for the indefinite. And the implication is also that all of those involved are mutually siblings, each to the other. Ergo: "his siblings" since we are discussing "He/Fred" his siblings.
Unless, of course "The Siblings" is a musical group, in which case it all goes haywire.
I'm not sure I understood your point; I never argued with "Fred is the least happy of the (3) siblings", just with "Fred is the least happy of his (2) siblings". But I think it's rather pointless to keep discussing the whys and whynots of this when that's clearly beside the point of the whole exercise.
I can agree with that. I just wanted to understand how a perfectly natural English phrase could be wrong.