Translation:My sister-in-law is a good friend of mine.
That would also work, but it would be redundant, since feminity is already expressed by the word "bofratino" and the word "amiko" is genderneutral.
I suspect that it has to do more with the irregularity of English than with Esperanto. Compare these phrases: his friend- a friend of his = his: his: her friend- a friend of hers = her: hers my friend - a friend of mine = my:mine In Esperanto it's simpler: de mi, de vi, de ŝi, de li ....so always the nominative.
The English is always the same kind of pronoun, too but it just happens that his in this case is identical to his when a noun follows but the rest are not.
That is only how it's done in English. In Esperanto mia would not fit as it would not be modifying any word. So really "of me" (de mi) is the way to go.
good friend of (among) mine (pl.) = bona amiko el/inter miaj.
bona amiko de mi = good friend of me (me's friend).
At least I feel so, but I can't explain :/ Using "de miaj" can be right (plural I think). It'll be understood anyway.
I agree with this feeling on the grounds that "bona amiko el miaj" can be heard as short for "bona amiko el miaj amikoj", where "bona amiko de mia" can't really be extended to anything sensible that I can think of.
Can someone tell me how to correct write "Cisco kid was a froend of mine" please. Thank you!!!
Is just a song from a band called War, back in the days. Should google it, perhaps you like it