The Torah forbids a man from marrying his wife’s sister as long as his wife is alive: “You shall not take a woman in addition to her sister, to make them rivals, to uncover the nakedness of one upon the other in her lifetime” (Vayikra 18:18). It seems that the Torah wants to make sure that sisters, who naturally love each other, do not come to see each other as enemies. However, if a wife dies, the Torah allows and even encourages the marriage of the surviving sister and the widower. This is because we can assume that in a household which lost its homemaker, the person most likely to be able to maintain a similar home environment would be the sister of the departed wife.
One of the seven Noachide laws is a ban on sexual immorality. Is marrying two sisters included in this prohibition? Some say that it is. When the Torah speaks of marrying two sisters, it uses the word “tikach” (take). This is the same verb used later in the Torah to refer to the mitzva of taking a wife. Thus they argue that the prohibition relates specifically to Jewish marriage (kiddushin), rather than to sexual relations. Kiddushin is a halachic framework relevant only to Jews but not to Noachides (non-Jews). Indeed, Ramban (in his commentary on Yevamot 97a) and many other Rishonim (medieval rabbis) see this as the reason that Yaakov was permitted to marry two sisters. Since the Torah had not yet been given, he was considered a Noachide.
However, others disagree. They point to the verse that introduces all the forbidden sexual relationships, “Any man shall not approach his close relative to uncover nakedness” (Vayikra 18:6). The verse is inclusive, with “any man” including non-Jews as well. Those who follow this opinion need a different explanation for how Yaakov was allowed to marry two sisters. One possibility, suggested by Ramban in his Torah commentary, is that as long as Yaakov lived outside the Land of Israel, he was not subject to the commandments, and, therefore, was permitted to marry two sisters.