If a person is under 18, mentally unable to make decisions for themselves, or physically unable to sign the form they can have a court-appointed representative to sign the form for them.  However, if your sister is active, she is not a veteran. She does not yet know the terms of her discharge.
Does this also pertain to my own adult children?
If you are applying for an unmarried adult child, you will also need to provide supporting documents about the child's disability.  You, as the veteran, should also ask the child's doctor to verify the documents.  The documents should include proof of the date of the disability's onset with a description of the disability, the marital status of the child, and a description of how dependent the disabled child is on you.