Fat Albert and the Cosby Kids (1972)

Beau and Francie are brother and sister. However, Beau lets it slip that Francie was adopted by their parents. After she confronts them, the parents indeed admit they adopted her when she was a baby. Francie is distraught and runs away, only to wind up in danger as she falls into a river and will get pulled into a drain tunnel. Beau and Fat Albert are able to save her in time and the family reunite proving that they love their daughter no matter what.