"And a man lie with her a lying of seed": a man, and not a minor. "And a man lie with her": and not with her sister. For it would follow (otherwise), viz.: If in a place where the forbidder [(i.e., her husband, who forbids her to all men)] does not forbid her all of his days, (for he can divorce her) — she forbids her forbidder (her husband) from living with her (if she secretes herself), then in a place where the forbidder (his wife) forbids her (sister to him) all of his days — how much more so should she forbid (to herself) her forbidder (if he cohabits with her sister)!
It is, therefore, written ("and a man lie) with her," and not with her sister. Abba Channan says in the name of R. Eliezer: "with her": and not (if he lay with) with his mother-in-law; "with her": and not with one of the illicit relations. For it would follow (otherwise), viz.: If one commits a (relatively) "light" act of forbidding, in that he does not forbid her all of his days — she forbids her forbidder, then if one commits a grave act of forbidding, in that he forbids her all of his days, how much more so should she forbid (to herself) her forbidder!
It is, therefore, written "with her," and not with his mother-in-law; "with her," and not with one of the illicit relations.