The nakedness of your mother’s sister you shall not uncover … (Leviticus 18:13)
We can take the entire previous mitzvah and just substitute the word “mother” for “father.”
As it does with a father’s sister, the Torah here says, “(because) she is your mother’s flesh.” Incest with a maternal aunt is forbidden for all of the usual incest reasons, plus because it’s an affront to one’s mother.
This mitzvah applies in all times and places. It is discussed in the Talmud in tractates of Yevamos (54b) and Kerisos (2b). It is codified in the Shulchan Aruch in Even Ha’Ezer 15. This prohibition is #341 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #130 of the 194 negative mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.