Musar su Genesi 20:12

Sarah, whose function was the perpetuation of the species, is therefore "Mother Superior" as per the words of Abraham to Avimelech in Genesis 20,12 אחותי בת אבי היא אך לא בת אמי. "she is my sister, the daughter of my father, but not the daughter of my mother." Sarah's distinction was rooted in the spiritual domain called אם. When compared to the spiritual domain known as אב, she is אחות and בת, but not אב. Once you appreciate the foregoing you will understand why the description אם כל חי, which was Adam's description for Eve, can be applied in equal measure to Sarah.

– neither could he be faulted for marrying Zilpah and Bilhah, though the latter were sisters of Leah and Rachel respectively. Since they were both only half-sisters, i.e. shared the same father with Leah and Rachel but not the same mother, and, according to Jewish law, we can never be certain of the paternity of pagans, they could not legally be considered "sisters" (cf. Rashi on Genesis 20,12 where Abraham explained to Avimelech that Sarah was his sister from his father's side). Should you want to know what relevance the question of Jacob marrying two sisters has, seeing that when a person converts to Judaism such a person has legally severed all former relationships, and the convert is considered as being a newly born person, we would have to answer that as long as the Torah had not been given, the Jewish people were part of the general human society and could not claim to be a nation totally distinct from all others. Leah and Rachel then were sisters from their mothers' side. Jacob did have reason to be concerned. G–d did not only accept Jacob's prayer, but it was Rachel's son יוסף הצדיק who became the antidote to Esau. The "mother" that Jacob had worried about as a potential victim of Samael, produced Samael's nemesis.

The Sages relate in Bereishis Rabbah (Parashas Bereishis 20:12) regarding the verse, “And God made tunics of leather [ohr, spelled with an ayin (עור)] for Adam and his wife” (Bereishis 3:21): “In the Torah scroll of Rabbi Meir it was stated, ‘tunics of light’ [or, spelled with an alef (אור)].” This Midrash is very perplexing. Why did the spelling, “tunics of light,” appear specifically in Rabbi Meir’s Torah rather in any other? [See what my illustrious father Rabbi Shmuel Kaidenover writes about this matter in Birkas Shmuel, Parashas Bereishis, 6c, in the name of the illustrious Rabbi Leib Zunz, z”l.]