We report two heterosexual sibling cases of mild non-specific myopathy, with bilateral occipital cortical dysplasia and diffuse white matter hyperintensity on brain magnetic resonance imaging (MRI). The histological examination of the muscle in the elder sister revealed non-specific myopathic changes and no reductions of alpha-dystroglycan and laminin alpha2 expressions. The characteristic findings in the occipital lobe on brain MRI in both cases suggested cobblestone lissencephaly. Disrupted structure of the glia limitans and pial basement membrane complex, by a cause other than an alpha-dystroglycan and laminin alpha2 abnormality, may be the cause of the cortical dysplasia in the sibling cases.