Complete Passuk
וַתֹּ֣סֶף לָלֶ֔דֶת אֶת־אָחִ֖יו אֶת־הָ֑בֶל וַֽיְהִי־הֶ֙בֶל֙ רֹ֣עֵה צֹ֔אן וְקַ֕יִן הָיָ֖ה עֹבֵ֥ד אֲדָמָֽה׃
Chabad / Judaica Press
Core Text
Core Textאֶת אָחִיו אֶת הֶבֶל — הֶבֶל וַאֲחוֹתוֹ, קַיִן וַאֲחוֹתוֹ.
English of Core Text: “His brother, Hevel” — Hevel and his twin sister; Kayin and his twin sister.
On the daf / in context: The doubled “es” is taken to indicate that each brother was born with a twin sister.
Type of Learning: Custom (twin sisters with Kayin and Hevel)
What Is Learned: Each of the first two sons was born together with a twin sister.