Question: How is it possible for Cain to have a wife if he, Abel, and Seth are the only children of Adam and Eve and they are the first people created by G-d?
Answer: Hi! Thank you for your interesting question. The Talmud ( Sanhedrin 58) asks this question too. The answer is from a difficult verse in Leviticus 20(17): “A man who marries his sister…it is a chesed...and the two are cut off from their people…” Now the word chesed usually means kindness, but the commentary of Rashi on the verse explains that there is an Aramaic form which means humiliation, and that is probably the simple meaning of the verse. The Talmud says that the word also has its more usual meaning: Once, G-d did a kindness for the world, allowing a man (Cain) to marry his sister, in order that the world should be build up. It seems that Adam and Eve had daughters who were not mentioned by name (see Genesis verse 5(4)). Cain and the others were allowed to marry them.