Susan Burns, a regular reader of Just Genesis and a fellow member of Open Anthropology Cooperative, has written an interesting and informative piece on Cain and Abel. She and I agree that the textual evidence indicates that Cain and Abel were twins. Here is what Susan has written:
Genesis 4: Adam lay with his wife Eve, and she became pregnant and gave birth to Cain. She said, "I have gotten a man with the help of YHWH". She again bare his brother Abel. The Hebrew word used for again (yasaph) is an adverb meaning to continue to do a thing. Yasaph implies that Eve gave birth to Cain and continued to do the same thing by giving birth to Abel. In other words, Cain and Abel were twins. The profession of Abel was shepherd and Cain was a farmer and city builder.
The tradition of twins as the progenitors of tribal units or city builders is very well documented in Semitic and Indo-European cultures. When birth order is specified, the younger twin always receives the blessing over the first born brother. In the account of the sons of Adam, the first born twin is envious of the second and commits fratricide. There are many variations on this theme in other twin genesis accounts. Jacob is fearful that Esau will kill him, Romulus killed Remus and Gwyn and Gwythurin in Celtic tradition duel every May.
The Gemini twins, Castor and Pollux, shared a mortal and an immortal existence. Castor was killed on a cattle raid but Pollux persuaded Zeus to allow the brothers to switch places periodically. The word Gemini comes from the PIE root *ym which means 'to pair'. This word is very similar to the Hebrew im mimation suffix but, of course, linguists say they are unrelated (sigh).