I knew a boy who died from that condition at a very young age. The parents had almost round the clock support although his was the most severe type.
If a child has it a later sibling has 25% chance of having it too. It is not hard to imagine 3/4 siblings being affected.
The children are mentally normal infact I have heard are sometimes known to be very bright. I know my friends boy loved anything visual and communicated very well through facial expressions.
I don't think a nanny and a maid was really the type of support this woman needed. Very very sad.