Did and Cain and Abel marry their sisters?

If you believe the Bible, then at least one of Adam and Eve's sons had to marry one of their daughters. Adam and Eve are the ancestors of all living humans (Gen. 3:20), which by definition means that the only women available at first would be sisters (in all likelihood, nieces would also become available to marry not long after the first sibling marriage).

Keep in mind that since the early humans lived up to around 900 years (about 10x modern age), and Adam had at least three sons by the time he was 130, if they averaged a child every other year until age 500 (which is conservative - Noah only got started at age 500), then Adam and Eve had 185 children - so "sister" would be genetically accurate, but relationally, you wouldn't think of them the same way we do.

Marrying siblings was not outlawed until Moses day - in the giving of the law (Lev. 20:17). Examples of marriages between close relations: Abraham married his half-sister Sarah - Gen. 20:12; Isaac married his cousin's daughter (Gen. 22:20-23); Jacob married his cousin (Gen. 29:10, 28); Moses' father married his aunt (Ex. 6:20).