> The forms 'minn kæri (faðir/bróðir)' "my dear (father/brother)"
> suggest that 'minn litli bróðir' might be acceptable after all.

The normal modern address is: "Litli bróðir!"
I suppose you *could* say: "Litli bróðir minn!"
But I'm not sure why the 'minn' is needed.

A concocted Old Norse form might be:
"Bróðir (minn) inn litli!" but I don't
recall seeing that anywhere.

Incidentally the use of weak adjectives with
nouns without article sort of creates a vocative
case in modern Icelandic.

Without article the words are declined like this:

nom. lítill bróðir litlir bræður
acc. lítinn bróður litla bræður
dat. litlum bróður litlum bræðrum
gen. lítils bróður lítilla bræðra

With the article the words are declined like this:

nom. litli bróðirinn litlu bræðurnir
acc. litla bróðurinn litlu bræðurna
dat. litla bróðurnum litlu bræðrunum
gen. litla bróðursins litlu bræðranna

But then we get:

voc. litli bróðir! litlu bræður!

And there is no word like 'sá' or 'þessi' to
turn the adjective weak.

Also incidentally I hadn't realized what an excellent
exercise declining "litíll bróðir" is. Several cute
little irregularities in both words :)

And on the subject of 'bróðir' I might mention that
some people use the same form (bróðir) for all cases
of the singular (same with 'dóttir' and 'systir').
They try to root out this "inferior" declension in the
schools and I think now it may mostly be heard from older
people.

Kveðja,
Haukur