The Torah in Parashat Ahareh-Mot (Vayikra 18:13) introduces the prohibition against having intercourse with one’s mother’s sister. As with regard to the prohibition against relations with one’s father’s sister, this prohibition applies regardless of whether one’s mother and this woman share two parents, only a father, or only a mother. In either case, relations with her are forbidden. Likewise, it makes no difference whether the mother’s sister was produced through a legal marriage, or through an illicit relationship; regardless, if they share the same father or the same mother, relations between the sister and her nephew are forbidden.
Needless to say, this prohibition applies in all places and in all times. One who intentionally has intercourse with his mother’s sister is liable to Karet; one who commits this transgression mistakenly must bring a Hatat (sin-offering).