It seems unusual that a tzaddeikes such as Sarah Imeinu would be so “mercenary” as to fear Yishmael’s inheriting Avraham’s possessions. Undoubtedly, as Rashi states, she was concerned about Yishmael’s evil influence over Yitzchak. Nonetheless, what is the meaning of Sarah’s statement, “For the son of that slave-woman shall not inherit with my son”? Horav Elyakim Schlesinger, Shlita, explains that as long as Yitzchak and Yishmael’s relationship did not involve monetary dealings, Sarah did not fear any harmful persuasion from Yishmael. Once they would begin sharing an inheritance, however, this distinction would have slowly diminished. Suddenly, Yitzchak would have been compelled to spend more time with Yishmael in order to make joint decisions regarding their material holdings. This increased fraternizing between the two brothers would have created a climate that would be spiritually detrimental to Yitzchak.
Indeed, Sarah was perceptive in regard to the harmful influence that Yishmael might have upon her son, Yitzchak. Horav Schlesinger suggests that this is what Chazal meant when they commented that Avraham was secondary to Sarah in regard to nevius, prophecy. Sarah was more sensitive than Avraham. She was able to foresee what harm would result from a relationship between Yitzchak and Yishmael. Whether we attribute her keen perception to experience or intuition, Sarah was able to sense what Avraham did not. Therefore, Hashem told Avraham, “Whatever Sarah tells you, heed her voice.”
It is logical for us to assume that the standards which were relevant for Sarah Imeinu as she raised the next Patriarch are similarly applicable today, as we raise our own children. We do not want to witness the harmful results of a non-Torah environment. It would serve us — and consequently our children — well to be just a little more perceptive, so that we benefit from foresight rather than hindsight!
