Rashi on Leviticus 18:18/רש"י על ויקרא י״ח:י״ח

1‎ אל אחתה ‎[NEITHER SHALT THOU TAKE A WIFE] TO HER SISTER — both at the same time.

2לצרר is connected with the word ,צרה — a rival (cf. I Samuel 1:6). Thus the verse signifies “thou shalt not take a wife to her sister", to make (i. e. making) the one the rival of the other.

3בחייה DURING HER LIFE-TIME — This teaches you that if he divorced her (his wife) he is not allowed to marry her sister so long as she (the former) is still living (Yevamot 8b).