I'm told that this word refers (or referred) specifically to the maternal uncle. Does anyone know of words denoting the maternal aunt, or the paternal aunt and uncle?

here's a URL with some of the information you want. You have to scroll down a ways.

http://www.theatlantic.com/unbound/fugitives/likeanaunt.htm

well, back in those heady Word Fugitive days, I didn't have access to OED; here's what it says regarding 'materteral'

a. humorously pedantic.

[f. L. mQtertera maternal aunt.]
Characteristic of an aunt. So ma"terterine a.

just as avuncular is used now generally for characteristic of an uncle....