Negative Commandment #128
Sefer Ha’mitzvos Ha’Katzar (The concise book of mitzvos)
By the Chofetz Chaim
This book lists the Torah mitzvos that can be observed today
Second Section – the Negative Commandments
128. It is a negative commandment "do not have relations with one's sister who is the daughter of his father's wife"
As it states in Vayikra (Leviticus) 18:11, "the nakedness of your father's wife's daughter... do not uncover." This means she is one's sister. If she was the daughter of one's father's wife from a different man, then she is permitted initially, but she is still his sister. Having relations with his father's wife's daughter requires two punishments, as explained in Negative Commandment 127. He does not receive the death penalty for transgressing this commandment, only spiritual excommunication, or if done without intention, he is required to bring a sin offering.
This applies in all places and at all times.