Guardianship

My parents are requesting gaurdianship of my 2 neices, who have pretty muched lived with them all their lives anyway. They are 6 and 8. My question is, what is legal guardianship exactly, what rights does it give my parents, and what rights does it give my sister and the father, which are not together. One father is deceased, and my sister gets SSI for my one niece, the other father pays support, and is backng my parents in this decision. When my sister is stable, will it take a court hearing for them to be released back to her? Im really confused about all of this, and honestly so are my parents, but looking to protect the kids. We live in Indiana by the way. Thanks