In Mo'ed Katan (Bab. 20a) the Gemara relates a story about how Rav learned of the death of his parents from a conversation with R' Hiyyah. The Gemara tells us their relationship, and it's pretty critical to understanding the story that we know they are related through Rav's parents, but I just can't wrap my head around what the relationship is. It sounds like the Gemara is saying that Rav's parents were brother and sister. Can someone please clarify this for me?
Rav, the son of the brother of R' Chiyya, was the son of the sister of R' Chiyya
דרבי אחא מכפרי נשא לאה והוליד ממנה אייבו מתה ונשא רחל ולה בת מאיש אחר וממנה נולד רבי חייא ונשא אייבו הבת ונולד להן רב ונמצא רב בר אייבו בר אחוה דרבי חייא מאבוה ובר אחתיה מאמיה
Rebbi Acha married Leah and had Ayvo. She died, he remarried Rachel and she had a daughter from a previous marriage, and from Rachel Rebbi Acha had Rebbi Chiyya. Then Ayvo married the daughter from the previous marriage and they had Rav. So Rav son of Ayvo is both the son of the brother and son of the sister of R' Chiyya.
Leah -----Rebbi Acha ------- Rachel | | \ Ayvo Rebbi Chiyya Daughter \ / \ ------------------- / | Rav
So Ayvo married his step-sister, and Rav is related to R' Chiyya through his father and his mother, both as half-siblings.