From Latin incestum "unchastity, impious unchastity," incestus "unchaste, impure," then Old French inceste "incest; lechery, fornication."
Therefore, the real meaning of incest is not limited to sex between relatives, but is much broader, and includes all forms of sexual misconduct.
In Kabbalah (and therefore the Bible), there are many forms of "incest," sexual misconduct.