The Mafu Cage (1978)

Synopsis
In this combination of domestic melodrama and offbeat suspense flick, an astronomer must shoulder the burden of caring for and sleeping with her mentally ill sister after their anthropologist father dies in Africa. Ellen (Lee Grant) and Cissy (Carol Kane) share a possibly incestuous relationship, but that doesn't stop Ellen from longing for a more conventional romance with nice-guy co-worker David (James Olson). And who could blame her for wanting to get away, considering that Cissy's favorite activity is drawing pictures of, then killing, the series of primates she keeps locked in a cage in the sisters' baroquely tribal abode? After sleeping with David and accidentally letting her resentment against Cissy show through, Ellen heads out on a business trip, convinced her sister won't come too unhinged as long as old family friend Zom (Will Geer) looks in on her every day. Unfortunately, David comes knocking, delighted to finally get some insight into Ellen's home life, and soon Cissy has trouble differentiating one kind of primate from another. Based on a French play, The Mafu Cage marked the second effort behind the camera for former actress Karen Arthur, who would go on to become an in-demand TV-movie director. ~ Brian J. Dillard, All Movie Guide