Why does the Torah need to inform us that Elisheva was Nachshon's sister?
Bava Basra 110a: Since most children turn out to be like their maternal uncles.
Gur Aryeh: Otherwise, it would be unnecessary to tell us that she was Nachshon's sister; we could have derived this from Bamidbar 1:7 (when we are told that Nachshon's father's name was also Aminadav).
QUESTIONS ON RASHI
Rashi writes: "From here, we learn that when marrying a woman, one must investigate her brothers." Why is this so?
Gur Aryeh: The nature of male offspring can be attributed to their mother. 1 (Therefore, if one wants to know what his sons will be like, they will usually resemble their mother's brothers - Bava Basra 110a). One cannot ascertain this by investigating the prospective bride herself (or her sisters); because they are women, this will not give the full picture. 2
Maharal (Chidushei Agados Vol. 3, p. 123, to Bava Basra 110a): Though it is true that sons resemble the mother, since the sons are male and the mother female, it is their maternal uncles whom they resemble.
Refer to 6:23:1.3:1.
Gur Aryeh: Conceptually, a female is comparable to an unfinished vessel, whereas a male is a completed vessel. When conducting a test, one should use a completed vessel. (This alludes to the Gemara, that a woman makes a covenant only with her (first) husband, who makes her into a vessel (Sanhedrin 22b)). Maharal (Chidushei Agados loc. cit., Vol. 3, p. 143) - That is how man and woman can become absolutely one. In Maharal's terminology (see Gevuros Hashem Ch. 56, p. 249), the female is Chomer, and the male is Tzurah; this is what creates the bond between them.) (EK)
Rashi writes: "From here, we learn that when marrying a woman, one must investigate her brothers." Why in fact is the nature of male offspring attributed to their mother?
Gur Aryeh (to Bereishis 35:8): It is the mother that gives the developing fetus its blood, flesh, etc. (see Nidah 31a). Its base physicality comes from the mother, to a greater degree than the father. 1
Also refer to Bereishis 46:15:1.2.
