We will quote Genesis 4:1-2 from the Authorized King James Version, the New American Standard Version and the New International Version.

“And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the LORD. And she again bare his brother Abel.” (AKJV)

“Now the man had relations with his wife Eve, and she conceived and gave birth to Cain, and she said, ‘I have gotten a man child with the help of the LORD.’  And again, she gave birth to his brother Abel.” (NAS)

“And Adam lay with his wife Eve and she became pregnant and gave birth to Cain.  She said, ‘With the help of the LORD I have brought forth a man.’ Later she gave birth to his brother Abel.” (NIV)

From the above translations as well as other translations, no direct evidence is given that Cain and Abel were twins. In instances in the Bible where twins are born, the events are clearly noted. (See Esau and Jacob [Genesis 25:24-26] and Perez and Zerah [Genesis 38:27-30].) In the case of Cain and Abel there is no clear statement that they were twins.  In fact, in most of the Bible translations it appears that Abel was born afterwards. “Later she gave birth to his brother Abel.” (Genesis 4:2 NIV)  Therefore, from the evidence we have, we conclude that Cain and Abel were not twins.