The Torah in Parashat Ahareh-Mot introduces a prohibition against having intercourse with “the daughter of your father’s wife.” If one’s father produced a daughter with a wife, then this command forbids relations with that daughter. A separate prohibition forbids relations with one’s father’s daughter even if she was produced by rape, meaning, if the father forced himself on a woman, thereby impregnating her, and she delivered a girl. If one’s father produced a daughter with a wife, whether or not that wife is this son’s mother, relations between the son and the daughter violate two prohibitions – the general prohibition forbidding relations with one’s sister, and this specific prohibition forbidding relations with the daughter of one’s father’s wife.
One who intentionally violates this prohibition, and engages in relations with his sister, is liable to Karet. If this was done unintentionally, then he must offer two Hatat sacrifices (sin-offerings), unless the sister had been produced through a forced relationship, in which case he must offer just a single Hatat, as only one of these two commands has been transgressed.
As mentioned in reference to an earlier Misva, Halacha does not forbid marrying a stepsibling. If one’s father married a woman who has a daughter from a previous marriage, the son may marry that daughter.