Your son is legally responsible for his daughter. Sounds like a terrible risk to leave this particular child home alone.
As a grandparent, you could file a guardianship to care for her until she completes High School.
Please note that this answer does not constitute legal advice, and should not be relied on, as each situation is fact specific, and it is not possible to evaluate a legal problem without a comprehensive consultation and review of all the facts and court pleadings filed in the case. This answer does not create an attorney-client relationship