YA fiction 1980s - adopted girl with paranormal abilities

I'm looking for a YA fiction book from the 1980s about a young girl (?Otilie ?Otalie) left behind by her adopted family after some sort of apocalyptic event. She makes her way across the US to a safehold in a cliff, rescuing a baby with a milk allergy on the way, where she gathers together a group of children with unusual paranormal abilities. They serve an obscure wheelchair-bound man. The girl discovers that her adopted family are actually her real family and her brother and mother join her in repelling her controlling and violent father with the aid of the other children. There is a medical genius who arrives in a plane, a young boy with an eidetic memory and a pair of twins in the family the protagonist gathers together.