Is the daughter of the maternal aunt by fosterage (who was nursed by the fosterage grandmother) considered unlawful for the questioner to marry, and is she considered his maternal aunt by fosterage?

It appears that your female cousin through

nursing, who was nursed by your mother's mother (your aunt through nursing), becomes unlawful for you [to marry]; because she became a sister to your mother through nursing and an aunt to you through her. Consequently, she is among your unmarriageable kin whom it is forbidden for you to marry, due to the saying of the Prophet, peace and blessings be upon him: "What is unlawful by lineage is unlawful by nursing."