Comentario sobre Génesis 4:2

רעה צאן A FEEDER OF FLOCKS — Because the earth had been cursed he refrained from cultivating it.

ויהי הבל רועה צאן, Hevel chose this vocation as it required more intelligence and involved one’s mental activities more that farming.

ותוסף ללדת את אחיו, She continued to give birth to his brother. The reason the Torah adds the words "his brother," something quite unnecessary, as well as the repeated use of the word את, has been explained by our sages. They said that in the struggle between Cain and Abel, the latter was first victorious, Cain lying on the ground. Abel then pitied him and moved away. Cain exploited this pity of his brother Abel to attack him and kill him (Tanchuma Bereshit 9). The words את אחיו are a hint that Abel acted in a brotherly fashion towards Cain. By placing the word את in front of the word הבל, the Torah alluded to the lack of brotherliness with which Cain related to Abel. Another reason for the word את both times before the name Cain and the name Abel is to indicate that twin sisters were born together with these brothers (Yevamot 62).