According to Rashi to Bereishis 4:1-2, Kayin was born with a twin sister, and Hevel was born with two triplet sisters. As Rashi to Vayikra 20:17 explains, Kayin married his sister in order that the world could be populated.
Which sister(s) did Kayin marry? Did he marry his own twin, or did he marry one of Hevel’s triplets?
Did Hevel marry before he was killed? If yes, which one did he marry? If no, which one was he supposed to?
What about Sheis? Rashi expounds each time את appears in 4:1-2 as a reference to another sister, but no such word appears in either 4:25 or 5:3 regarding his birth (though it does appear in both regarding his name). Since he therefore wasn’t born with a twin, did he marry Kayin’s twin sister or did he marry one of Hevel’s? Or perhaps he married a female descendant of Kayin’s?
These questions are specifically in Rashi’s opinion that Kayin married his twin sister. If there are alternative explanations as to whom each brother married that might possibly fit into Rashi’s opinion, I will only accept such an answer if you can demonstrate that Rashi holds that way as well.