Thank you for your advice! Please see my additional comments above with a question.
If your sister has legally adopted, she is in control of the circumstances under which you will have contact, if any.
Beings that she is in control of my children, would she be in violoation of any sort if she assigned temporary custody to me (is she even able to) so they can attend school in PA? They are so close to being adults, 16 and 13. This is such an important time in their life and they need their father.