Is there a feminine equivalent to the adjective "avuncular"?

Solution 1:

Deriving from your own explanation in the OP, the natural choice would be amicular.

I do not seem to find any dictionary entries. Need to see why.

Preliminary:
Book Doctor Gwen : 92 Feminine and Masculine Word Pairs
Feminine term / Masculine term /// neutral or inclusive term
4. amicular* / avuncular
(*Terms that are slang or recently coined.)

Contemporary Pragmatism - Google Books Result
books.google.com/books?isbn=9042018445...
John R. Shook, Paulo Ghiraldelli - 2004 - Philosophy - 200 pages
... be offered as amicular advice to discourse generation researchers, along the lines of the earlier 'Don't ask for the meaning; ask for the use', ...

Solution 2:

The correct answer, courtesy of the Oxford English Dictionary, is the word materteral, whose entry I give here in toto:

Pronunciation: Brit. /məˈtəːtərl̩/ , U.S. /məˈtərdər(ə)l/

Etymology: < classical Latin mātertera maternal aunt ( < māter mother n.¹ + ‑tera , feminine of ‑ter, suffix forming nouns) + ‑al suffix¹.

humorous. rare.

Characteristic or typical of an aunt. Cf. avuncular adj.

• 1823      W. Taylor in Monthly Rev. 102 447      With maternal and materteral anxiety.
• 1867      J. N. Taylor Spindrift 6      You can picture the stately materteral form—A full-blown Atè, big with doom!

The proper citation for that entry is:

materteral, adj.

Third edition, March 2001; online version December 2011. < http://www.oed.com/view/Entry/114954 >; accessed 18 February 2012. An entry for this word was first included in New English Dictionary, 1905.

Solution 3:

While their lyrical nature is subjective, dictionaries list both aunt-like as well as auntly as suitable adjectives.