Sibsex is when you feel sick, mentally, emotionally and, in extreme cases, physically, from watching your siblings have sex.
Some people are immune to this. They are called Hilljacks (Hillbillies, Rednecks, etc.). Their biological children are often also their nephews and nieces.
Joe often experiences a bad case of sibsex (he feels bad, starts to get a headache, and eventually throws up) while watching video of his sister getting some. His older brother, who is a hilljack, often watches the video (when their sister is out of town, and not available) and forces Joe to watch along with him, for company.