About the older girls... good on his Mama. Didn't it used to be older boys who tried to debauch younger girls? It seems things have gone backwards in that regard, at least in my experience.
I'm not advocating sexual relations among the mentally retarded. However... and I know this sounds crass... they still have the primal urge to breed... but they don't have the capacity to understand the consequences. Unless they are kept under constant surveillance, opportunities will arise, most often with others with the same condition.
People who have not been around challenged people may not understand. Retarded people are not generally kept in state schools or mental hospitals. More often than not, they live at home with their parents, they often have jobs of some kind, and are "mainstreamed" into normal society as much as possible. Sometimes, and it looks as though this is the case, they form loving pair-bonds. These people don't live under a microscope where they are constantly monitored.
Common sense says that responsible caregivers should be given some leeway in protecting them from disastrous pregnancy.
I know we tend to bash the judiciary around here, but in reading what the judge had to say, it sounds as though she rendered a reasonable decision based on the facts of this individual case.