Actually they were a really special case, which I love, apologies if you're not interested at all but indulge me if you can.
What happened with them was a combination of factors that came together, not related to abuse as int he case of Gennie. Firstly, from birth they were expected to have some kind of intellectual disorder and slower development meaning certain aspects of their behaviour were not corrected or paid attention to. Secondly, their parents spoke English but they were mostly looked after their grandmother who spoke German and didn't interact with them a lot. In addition the girls were not sent to school or interacted with other children.
This in part led to the development of their own language, something common with twins, it's called cryptophasia. It happens because you have two children at the same stage of linguistic development who can work within the framework of adult grammar without having to figure it out completely. In addition, often parents of twins are much more stressed and as a result the quality of adult interaction and language use in the household declines. It's one of the reasons twins tend to have slower linguistic development.
In the case of Poto and Cabengo (which is what the girls called each other) when their father mentioned to his government case worker why the girls were not in school he was told to take them to speech therapy where it was discovered that they had absolutely normal intelligence and difficulties in learning were the result of neglect. The girls language was a mixture of German, English and neologisms and the interesting thing is that it had a clear underlying grammatical set of rules - it wasn't just incoherent babble. They spoke with a strong stoccato rhythim which you can hear in this video and transferred to their English.
They did learn English, they said they remembered their own language but their father forbade them from using it. The parents neglect clearly had an impact on the girls however and they spend most of their adult lives doing menial jobs.