I’ve read my Bible often and carefully, and I’ve never found a prohibition against cousin marriage in Scripture. If you think about it, EVERYBODY is our cousin in some form, since we all ultimately have the same grandparents in Noah and Adam. So prohibiting cousin marriage, combined with all the actual Scriptural prohibitions, would’ve prohibited marriage completely.
Jacob’s wives Leah and Rachel were his first cousins (daughters of Laban, his mother’s brother).
Have you considered that traditionally, Uslam emerged with Ishmael and Esau, and this inbreeding has highly concentrated the bloodline if these two?