Was Yitro descended from Avraham Avinu, and if so, how? I seem to remember he was a descendant of Hagar/Keturah but can't find where I saw that.
This is the opinion of Rabbeinu Bachya/Bechayei to Shemos 18:9, who says that Yisro was a grandson of Avraham, through Avraham's son, Midyan:
וכשם שהיו עשרה דורות מנח ועד אברהם כך תמצא עשרה דורות מחם בן נח עד מדין בן אברהם ועשרה דורות ממצרים בן חם עד יתרו שהוא מבני מדין
Edit: It's also found in a midrash on this same verse.
Edit: and here's a source sheet from a shiur on it
Edit: this shiur quotes Maharatz Chajes/Chiyyus holding like this in his commentary to Nedarim 32b.
Also, see this discussion in Birkei Yosef 260:2, which also quotes Maharsha on the topic.
(Note that various other mefarshim and midrashim disagree.)