I'll preface this post by admitting that it's a long shot that people will have the answer to my question. It was a long time ago and all concerned are now dead (except me).
I grew up knowing I had an older sister who was being cared for by nuns in a residential home/hospital because she had something wrong with her and my mother could no longer cope.
She was 10 years older than me, and I never met her. She died aged 20 when I was 10.
My mother never discussed her, but mentioned her fondly, and would go to visit her. Other older siblings and aunts said that my sister never learned to speak or walk, and stayed at the development stage of a baby, although her body grew and matured.
When I was born my mother, who was a widow, could no longer cope and the local priest said he could find a home for my sister with an order of nuns who were also nurses.
Recently, when doing my family tree, I got a copy of her death certificate. The cause of death was listed as epilepsy and 'imbecility'.
Knowing what we do now about conditions which affect children - what would a modern diagnosis be for a child who never learned to speak or walk and who (in the words of my aunt) 'just sat on a chair like a beautiful wax doll'.