Adam Clarke on Genesis 4:2

And she again bare his brother Abel - Literally, She added to bear (ותסף ללדת vattoseph laledeth) his brother. From the very face of this account it appears evident that Cain and Abel were twins. In most cases where a subject of this kind is introduced in the Holy Scriptures, and the successive births of children of the same parents are noted, the acts of conceiving and bringing forth are mentioned in reference to each child; here it is not said that she conceived and brought forth Abel, but simply she added to bring forth Abel his brother; that is, as I understand it, Cain was the first-born, Abel, his twin brother, came next.

Abel was a keeper of sheep - Adam was originally a gardener, Abel a shepherd, and Cain an agriculturist or farmer. These were the three primitive employments, and, I may add, the most rational, and consequently the best calculated to prevent strife and an immoderate love of the world.