Why did Sarah ask for Hagar to be expelled together with Yishmael?
Seforno: Refer to 21:9:2:2.
Ramban: Because the young Yishmael would not have managed to survive without his mother.
Base on the previous verse (21:9), we would expect Sarah to demand, "he must not jest with my son (Metzachek);" why does she say instead, "he shall not inherit"?
Gur Aryeh #1 (to 21:9): Sarah was not so concerned that the righteous Yitzchak would be influenced by Yishmael's deeds; she was concerned mainly that Yishmael might kill Yitzchak over the inheritance.
Gur Aryeh #2 (to 21:9): An alternate explanation (unlike Rashi) - Sarah asked to banish Yishmael due to his evil deeds. Lest Avraham hesitate due to the prohibition of shortchanging a son from his rightful inheritance, she declared (prophetically) that Yishmael was unfit to inherit at all.
What made Sarah declare with certainty that Yishmael would not inherit Avraham?
Seforno: Because he was the son of a slave, and if one of the parents is Pasul, the child's lineage is that of the Pasul 1 - rendering him a slave, who is not considered an heir.
As we learn in the Mishnah in Kidushin (3:12)
Why did Sarah insert the second "Im," which is seemingly superfluous?
Rashi: What she meant is that Yishmael would not be worthy to inherit with Yitzchak, even if Yitzchak was her son but not as proper (righteous), or even if he was proper as Yitzchak but not her son, and how much more so 1 now that he was both her son and Yitzchak!
Targum Yonasan: Expel Yishmael, Sarah was telling Avraham, a) because he is not fit to inherit with Yitzchak and b) so that he should not wage war with him over the inheritance.
Gur Aryeh: The extra word "Im" teaches us that each factor would be enough independently.
QUESTIONS ON RASHI
Rashi writes: "He was quarreling with Yitzchak regarding inheritance." That would imply that Yitzchak had already grown somewhat. If Yishmael was 14 years older than Yitzchak, he must have been at least 17 when banished; if so, how could Hagar have carried Yishmael on her shoulder (21:14)?
Gur Aryeh #1 (to 21:9): We cannot estimate the earlier generations by today's standards.
Gur Aryeh #2 (to 21:9): Perhaps Hagar carried only the bread and water on her shoulder literally, while Yishmael leaned on her for support.
Rashi writes: "Yishmael would take his bow, and shoot arrows at [Yitzchak]... and say, 'I am only joking!'" (refer to 21:9:1:4). How is this derived?
Gur Aryeh (to 21:9): Why did Sarah mention only the matter of inheritance, and not the bad influence of Yishmael's sins? Sarah was mainly concerned that the fight over the inheritance was liable to result in bloodshed; she was not as concerned that Yitzchak would be influenced. 1
Gur Aryeh: It seems that according to Rashi, independent of how we interpret "Metzachek" in the previous verse (21:9), all agree that Yishmael was trying to kill Yitzchak over the inheritance. Refer to 21:10:1.3.
Rashi writes: "'With my son, with Yitzchak' - since he is my son, even had he not been as worthy as Yitzchak." Had Yitzchak not been worthy, what would be his advantage over Yishmael?
Mizrachi: Rashi means, had Yitzchak not been as worthy as he actually is - but certainly not were he to be a Rasha.
Gur Aryeh: Yitzchak would take precedence, as son of the mistress of the house, while Yishmael was the son of the maidservant.
