Basic idea is some kind of family tradition/ritual where younger members challenge their parents to prove they're old enough to beat them. Fights include wrestling, boxing, hairpulling and sexual grabbing/punching for both male and female couples.
Another idea could be father and son fighting for the right to fuck mother and sister, either a cockfight or first to cum match.
No mixed matches, only Father vs Son and Mother vs Daughter. Anyone interested and willing to write it?