Parashas Vaeira
וַיִּקַּ֨ח אַהֲרֹ֜ן אֶת־אֱלִישֶׁ֧בַע בַּת־עַמִּינָדָ֛ב אֲח֥וֹת נַחְשׁ֖וֹן ל֣וֹ לְאִשָּׁ֑ה וַתֵּ֣לֶד ל֗וֹ אֶת־נָדָב֙ וְאֶת־אֲבִיה֔וּא אֶת־אֶלְעָזָ֖ר וְאֶת־אִֽיתָמָֽר׃
Aaron took to wife Elisheba, daughter of Amminadab and sister of Nahshon, and she bore him Nadab and Abihu, Eleazar and Ithamar.
Torah Temimah points to Bava Basra 110a (verified).
אחות נחשון — ממשמע שנאמר בת עמינדב איני יודע שהיא אחות נחשון? ומה ת״ל אחות נחשון — ללמד שהנושא אשה צריך לבדוק באחיה.
English: “Sister of Nachshon” — from the fact that it says “daughter of Aminadav” do I not know that she is the sister of Nachshon? Why does Scripture say “sister of Nachshon”? To teach that one who marries a woman must examine her brothers.
Custom
Chazal derive that a prospective husband should investigate the character of his wife’s brothers.