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In Bereishit 28:9, Rashi uses the words "אחות נביות" ("the sister of Nevayot") to calculate how old Yishmael, Eisav, and Yaakov were at the time, and a bunch of other dates.
However, in Shemot 6:23, Rashi says that "אחות נחשון" ("the sister of Nachshon") mean that one should find out who his brother-in-laws will be before getting married.
Why does Rashi interpret these two cases so differently?
Had there been extra pesukim showing how long Aminadav lived and when Aharon took Elisheva as a wife and when Elazar married bas Putiel, then we could have made a drasha like that. However, in this case, all we know is that Aharon married some time during the galus mitzrayim and that Pinchas was born some (unknown) time before yetzias Mitzraim and that he was old enough to be active and young enough to still kill Zimri and lead the Bnai Yisrael against Midyan (and kill Bil'am).
Any attempt to make a drasha similar to that which is given by Eisav would therefor fail for lack of information. Rash is pointing out that given the information in the Torah, and the way it is presented, this is the point that we can learn from it.
