Family law, child custody by family member, sibling, in another state

Although the law may vary slightly in VA, generally speaking, you can have a guardianship over your sister, so that you are responsible for her, and can make decisions (enrolling her in school, getting her medical attention, etc.) on her behalf. Mom just has to sign the paperwork agreeing to the guardianship. Go to your local courthouse and speak with the self-help people, or seek legal advice from an attorney who handles Probate matters. Good luck to you!