ummmm Good morning my darling. Almost certainly NBD.
Arnold-Chiari malformation various types depending upon the severity. It is a bony abnormality between the base of the skull and the first cervical verterbra, which might or might not be associated with neurolgical symtoms.
As I said, varying degrees, but may often remain symptomless.
MRI scans of the base of the skull will determine severity.
I understand the worry that surrounds all involved, but it is likely to be not as bad as thought. In simple English, NBD until proved otherwise.