At first glance, this mitzvah is bewildering. Leviticus 18:9 already prohibited relations with a sister, whether on the mother’s side or the father’s (see Mitzvah #192). What does this verse come to add?
As we have stated, there are some prohibitions that “double up.” For example, if a person born out of wedlock has incestuous relations with his mother, he violates Mitzvah # 190. If a person has relations with his stepmother, he violates Mitzvah #191, the prohibition on a father’s wife. If one has relations with his mother who was married to his father, he violates both of these injunctions. Similarly, if one has relations with a sister on his mother’s side, he violates Mitzvah #192. If he has relations with a sister on his father’s side, he violates both Mitzvah #192 and this mitzvah.
But there’s an important limitation! This verse only pertains to “the daughter of your father’s wife.” In other words, the mother has to be someone the father is qualified to marry. If the girl’s mother is forbidden to the father, incestuous relations would violate Mitzvah #192 but not this mitzvah. The result is that it is more stringent to have illicit relations with a “legitimate” sister than with an “illegitimate” sister (though both are prohibited). Now the apparent redundancy actually makes more sense! (See the commentary of Nachmanides on Leviticus 18:9 for more on this.)
We’ve already explained the basic rationale underlying the prohibition on incest in the previous mitzvos. One is permitted to marry an actual stepsister, that is, the daughter of stepmother from a different father. This is true even if they were raised as siblings. (See Rambam Hilchos Issurei Bi’ah 2:3 and Shulchan Aruch Even Ha’Ezer 15:11.)
This mitzvah applies in all times and places. It is discussed in the Talmud in tractate Yevamos on pages 22b-23a. It is codified in the Shulchan Aruch in Even Ha’Ezer 15. This prohibition is #333 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #128 of the 194 negative mitzvos that can be fulfilled today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.