Yes, sex is occurring between genetically / biologically related people, but GSA is something much deeper that that.
They are fully briefed by the CIA when they are occurring.
Unfortunately, the issue is almost completely ignored in the U.S., even though it is occurring here as well.
1520s, "meet, meet in argument," from Middle French occurrer "happen unexpectedly" or directly from Latin occurrere "run to meet, run against, befall, present itself," from ob "against, toward" (see ob-) + currere "to run" (see current (adj.)). Sense development is from "meet" to "present itself" to "appear" to "happen" ("present itself in the course of events"). Meaning "to come into one's mind" is from 1620s. Related: Occurred; occurring.