Our Gemara on amud aleph offers advice on how to determine a suitable mate based on lineage:

Rava says: One who marries a woman needs to first examine her brothers so that he will know in advance what character his children will have, as it is stated: “And Aaron took Elisheva, the daughter of Amminadav, the sister of Nahshon” (Exodus 6:23). By inference from that which is stated: “The daughter of Amminadav,” do I not know that she is the sister of Nahshon, as Nahshon was the son of Amminadav? What is the meaning when the verse states: “The sister of Nahshon”? From here one learns that one who marries a woman needs to examine her brothers. The reason is as the Sages taught: Most sons resemble the mother’s brothers.

I have always assumed that the dynamic described was a combination of genetics, personal qualities, and family middos.  For example, a strict parental attitude could be helpful for one child and harmful to another. So, the brother of one’s wife, who has her genetics and his gender, might be a crystal ball to determine what his sons will be like.

Sefer Daf al Daf discusses an observation from Agra Dekallah (Toldos).  Yitschok married Rivkah despite knowing the sinister qualities of her brother Lavan. However, this is because it was determined via divine signs that they were meant for each other. Yet, how could Yaakov marry Rachel and Leah, whose kin were presumably nefarious? The answer given is that based on Rashi and the Midrash (Bereishis 30:27), Lavan did not have sons before Yaakov’s arrival.  Therefore there were no brothers, and Yaakov could be free of this concern.  

If this answer is true though, it adds a different dimension to this Chazal.  The brothers are not correlation, but causation. Meaning, virtuous or evil brothers will influence their nephews.  This idea is supported from a story of the Chazon Ish (Maa’seh Ish 6) who told a student of his who was visiting with his nephew to be a proper role model, and he quoted our Gemara.  Assuming that it was not meant as merely a melitzah, we see the Chazon Ish also believed this to be causation and not correlation.

Uncles have important work to do. Indeed, sometimes an uncle has a special way to connect to his nephew because he is at once paternal enough, but still not as strict or complicated as a parent.  Uncles can offer a unique kind of nurturance and acceptance that is both parental and separate. It is kind of half-way between the spoiling of a grandparent, and the heaviness of a parent.  Family is a good thing.

Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation

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