I followed the link to the test. Although I couldn't see the test itself, I was able to locate the answers and then some further discussion (which, unfortunately, just makes everything worse):
This question asked whether it was possible to ascertain the sex of
Evelyn from the following sentence:
“I should like to introduce you to my sister Amanda, who lives in New
York, to my brother Mark who doesn't, and to my only other sibling,
As certain readers pointed out, this sentence was in fact ambiguous,
suggesting an impossible scenario in which the speaker had two
brothers who were both called Mark, and an "only other sibling" called
Evelyn. Having sifted through the readers' comments, Mr Gwynne says
the sentence should have read “I should like to introduce you to my
sister Amanda, who lives in New York, to Mark, my brother who doesn't,
and to my only other sibling, Evelyn."
Gwynne explains: "The absence of a comma before 'who doesn't' makes
that clause part of the definition of 'Mark, my brother', implying
that there are other brothers. A comma after the words 'my brother'
would mean that there was only one brother."
I find the discussion of one or two brothers named Mark to be a complete red herring when it comes to the gender of Evelyn.
Further, the "implication" of something isn't a sufficient argument for something definitive when it comes to grammar, and no matter how many times I try to understand how a comma after my brother would necessitate there being only one brother, I just don't see it.
In short, I believe the statements of the people behind this test are simply wrong—as are the further comments provided by Gwynne. I would not follow this site as an authoritative source.
Edit: Thanks to James for providing an answer about restrictive and non-restrictive clauses. That partially explains what Gwynne was trying to get across. Unfortunately, it's still not correct.
Suggesting that the meaning of a specific part of a sentence necessarily determines that of the entire sentence is a false premise. At best, if one part contradicts another part, it means that the sentence needs to be rewritten.
Here is my sister, who lives in New York, my brother who doesn't, my second sister, who does, and my second brother who doesn't.
You can't claim that there's only one sister and one brother just because a restrictive clause has been used. The most you can say here is that the person has at least two sisters and two brothers—and that they should have used some additional commas in the expression of that fact . . .