The Rat wants a feminine equivalent of ‘avuncular’. That’s easy: ‘materteral’. According to the Random House Word of the Day site, the word is listed only once in the Oxford English Dictionary, but is actually older (1823) than ‘avuncular’ (1831). They also note that Latin had different words for aunts and uncles on the father’s and mother’s side of the family: you father’s brother and sister are your patruus and amita, your mother’s are your avunculus and matertera.
They do not note that the etymologies of three of these are transparent: your patruus is a second father (pater), your matertera a second mother (mater), and your avunculus a lesser grandfather (avus). In Anthropology and Roman Culture. Kinship, Time, Images of the Soul (tr. J. Van Sickle, Johns Hopkins, 1991), Maurizio Bettini gathers the evidence that the father’s siblings were “were expected to maintain an attitude of discipline, harshness and aloofness”, while the mother’s were stereotypically “warm and affectionate even to the point of indulgence” (both quotations from Matthew Slagter’s review, here). It appears that The Rat will soon be an amita rather than a matertera, but ‘amital’ is not an English word, and I imagine she’s planning to be more materteral anyway. I recently learned that I will soon be a patruus magnus (great-uncle), but I plan to defy the etymologies and be greatly avuncular.
