The Torah in Parashat Ahareh-Mot (Vayikra 18:12) forbids engaging in intimate relations with “Ahot Abicha” – “your father’s sister.” This prohibition applies even if the woman is the father’s half-sister, regardless of whether they share the same father or the same mother. Moreover, even if the sister was conceived out of wedlock, nevertheless, since she and her brother share the same father, relations between her and the brother’s son are forbidden. This prohibition applies in all times and in all places. One who intentionally transgresses this prohibition is liable to Karet, and one who commits this violation unintentionally must offer a Hatat (sin-offering).