Before the airing the episode where Kenny and his brother and sister are sent to a foster home, what animated sitcom extended again until 2016, taking the show to 20 seasons?
South Park (season 15):  Shortly before the airing of the season finale episode "The Poor Kid", "South Park" was extended again until 2016, taking the show to 20 seasons. The Poor Kid:  In the episode, Kenny and his brother and sister are sent to a foster home after police discover a meth lab in their house.
South Park