The Torah relates: “And Aharon took Elisheva the daughter of Aminadav, sister of Nachshon, as his wife and she gave birth to Nadav and Avihu, Elazar and Itamar” (Shemos 6:23).
Rava, in the Gemara Bava Basra 110a,
learns from this verse that “one who wants to marry a woman must
check into her brothers, as the verse states ‘And Aharon took Elisheva the daughter of Aminadav, sister of Nachshon.’ It is implicit from the fact the verse states she was the daughter of Aminadav that she is also the sister of Nachshon so why did the Torah need to spell out that she was ‘the sister of Nachshon?’ From here we learn that one who wants to marry a woman must
check into her brothers. A Braisa teaches that most
sons are similar to the brother of their mother.” Rashi
adds that “because she was the sister of Nachshon, the tribal leader of Yehuda that prominence is why [Aharon] married her.” The Maharsha
explains that “this concept make sense because brothers and sisters, being born from the same womb, will have a greater chance of having the same nature since they came out of the same mother, as it is taught in the gemara, Tractate Nidda, that most of the body of a fetus is created from the mother, therefore most of the time he will have the same nature as his mother’s brother, whether bad or good.” (Click here
for Hebrew text)
Based on this gemara and the Maharsha, it would seem that one is born with certain personality traits based on his or her genetic makeup, just as the color of one’s hair or eyes is determined by genes. For that reason Aharon married Elisheva, the sister of Nachshon, who was the Prince of the Tribe of Yehuda and the first person to walk into the Red Sea (even before it split), thereby showing unparalleled trust in Hashem. Earlier in this piece the Maharsha also pointed out (based on a Medrish) that Esav, who everyone knows was evil, came from Rivka whose brother was the evil Lavan. Yet this is not an ironclad guarantee, as we see from Yitzhak’s other son, Yaakov. Ultimately, being born with natural bad tendencies is also not a guarantee that one will live a life of sin, as ultimately everyone has bechira [free choice], and an individual born with the genetic predisposition for evil can work on themselves to become a good person. Indeed, even one who is naturally good might slip up and stray – however, it remains likely that one can indeed predict the likely personality traits his children will be born with based on his wife’s brother. This should logically mean that one should look for a wife whose brother is in good standing (unless she is the best choice in the world, as in Yitzhak’s case).The question is why; who cares what your brother-in-law is like, as long as you are compatible with and attracted to each other. She has a fine personality which you enjoy, and you get along with her; what more do you need? You see eye to eye with her on many issues concerning raising a family, including having a proper and advantageous home which each of you can see raising children in; why does anything else matter?
It would seem that family and the continuation of fine moral values is so central to Judaism that Rava suggests one must look into the brother of one’s perspective wife since most of the time his children will have the same personality, and in order to more likely raise children of fine character it is better to marry someone whose brother has fine moral character, just as we test for genetic diseases before getting married.
Ultimately, it is up to the parents on how they raise their children, and up to the children whether they choose to follow in their parent’s footsteps, no matter their genetic predisposition, as we see in the contrast between Yaakov and Esav. However, it definitely helps, and might ease the burden a bit, to take into account when looking to get married, a brother-in-law with good yichus [a highly touted background] as Aharon did.