My Dad was under guardianship of his Mom and now she has Alzheimer's. Who gets guardianship?

My father's elderly mom is in a nursing home and my fathers step sister has guardianship of her. Does my father guardianship automatically fall under his step sisters care? I never got any court order from courts in regards to appointing her guardianship. My father has been in a nursing home since 2003 and his step sister has only been to see him twice. Once in 2003 and once in 2006. His mom no longer lives in this area. She was moved to a nursing home to be closer to his step sister. I have been in DOC for 1 year and I'm wondering if/how my chances are at gaining guardianship of my dad.