You probably don't want this/my answer here, but since your questions are so often blanket generalizations about birth order, I can't help but answer (when I'm looking for something to answer. :) ).
Anyway, as far as I'm concerned (and as I've so often said one place or another), families and individuals and cultures are all so individual AND things in families change with time; so I don't know of any "sterling" quality that anyone "can never possess" under the right circumstances and in the right family (or at least with one "right" parent who knows what she's doing and how to nurture "sterling" qualities.
I'm a middle of three (well spaced siblings), so the one thing I can say about that is that I had my time to be the youngest in the family until my younger sibling was born. Other than that, with an older sister and younger brother, I, personally, will never be an older sister to a younger sister (from my two parents) or a younger sister to a brother. That's it.
I do think that when one shares a mother and father with siblings (and in this case, a middle child - me), and one has one brother and one sister, one tends to be less likely to view people like siblings-in-law and/or friends as "sister" or "brother". That doesn't mean one can't have a friend or two with whom one may be closer in any number of ways; but as a "middle" with one of each, and with "decent" spacing, I do think one views siblings as a "completely separate and unique relationship" - and even the closest of friends as yet a whole other kind of close relationship.
Of course, maybe the fact that I don't think in terms of the word, "sterling", is a sign that I am, in fact, a middle sibling. No offense, but from where I am I tend to think that the thinking in terms of "sterling" is often associated with first-borns, only kids, and maybe SOME people who are the only one of their sex among siblings of the other sex. Well, my Irish and Scottish working-class parents worked hard to make sure nobody thought in terms of "sterling". (lol) They just kind of took for granted that good values, nature, and behavior were expected - not "sterling". (I've strayed some from the question, as usual; but I always keep a copy in case whoever asked the question deletes it; so feel free, needless to say...)