הלכה על יבמות 108:16

From the laws of the commandment is, for example, that which they, may their memory be blessed, said (Yevamot 54b) that the prohibition of the sister of the father is whether she is the sister of his father from the father only or from the mother only; whether she is the sister from marriage or whether from licentiousness - as each one of these is called his sister regardless. And there is no room for secondary [prohibitions] here.

From the laws of the commandment is that which they, may their memory be blessed, said (Yevamot 54b) that she is called the sister of his wife, whether she is the sister from her mother or the sister from her father; whether from marriage or whether from licentiousness. And they also explained that the prohibition comes into force from when he designates her, and as the expression of taking of a woman is always understood by us, as we said. And also that which they, may their memory be blessed, forbade the sister of his bound one, as a fence for this prohibition. And the understanding of his bound one is such a case as one who has a brother and he dies and does not leave children, such that he is obligated by the Torah to marry his wife or to release her, as it appears explicitly in the written commandment (Sefer HaChinukh 597, 599). And that is the woman that is bound to the brother, meaning to say that she is in the domain of the brother to marry her if he wants. Since she is acquired by him from the Heavens, she is called, "his bound one." And they, may their memory be blessed, forbade (Yevamot 41a) him from marrying her sister from when she is bound to him, even though he did not designate her and did not do any action.