Poto and Cabengo (1980) review

Six-year-old twins Grace and Virginia Kennedy grew up in an odd environment - they were deemed to have developmental difficulties and prevented from playing with others their own age, their parents were poor, their grandmother spoke German to them, etc. - so they developed a "special language" (an "idioglossia") to communicate with each other that brought them a lot of media attention, but linguistics experts at the Children's Hospital in San Diego spent time working with them to get them up-to-date on using proper English.  This subject brings out documentarian Gorin's paternal side - he takes them to the zoo and prepares meals with them - but it's still a slight project: once it's explained what the girls are "saying" (which is fundamentally repetitive), the "mystery" dissipates and it has nothing else to offer.  In an interview, the director claims the kids are engaging in a "subversive act which has not been authorized by any social or ideological establishment" ... but to me it seems like two sisters are attempting to "bond" in the only way they know how.  The Kennedys were revisited some decades later and both had menial jobs, bringing up the age-old debate regarding "nature" versus "nurture"....