I believe that the principle is that you still declare her as non-accompanying and then provide a letter of explanation to say that that the mother has sole custody (explain why, referring to the legal situation for your country). As a result of having no custodial rights, you are not able to provide any documents or a medical examination. IRCC should then be able to exclude her from the application, although you may have to sign a document accepting that you can NEVER sponsor her, even if something happens to her mother.