PLEASE READ How can a person gain custody of a child that is not biologically theirs?

I'm needing some insight on non-parent custodial rights. My fiancé's step sister had a baby 3 months ago. She has no job(doesn't get assists), lives with her parents, doesn't have any means of transportation and other wise, has no ways to provide for my nephew. Her dad and my fiancé's mother are not legally married but have been together for over 12 years. My fiancé's mom and her father provide everything for my nephew. She is contemplating moving in with friends that are also not stable; and although my fiancé's mother says that she will not take the child; there is no legal way she can keep her from taking him. She's 20 years old, graduated high school in May, not planning to be pregnant(obviously), still has the mindset of a 15 year old. I don't think my nephew is in any immediate danger in her care as of right now bc she lives with her father and step-mother, which they are there monitoring her. She has no mother-baby bonding with him. I take him every weekend, out of my own love and care; from Thursday night to Sunday night. We have such a strong connection and everyone is starting to see that. He would honestly be in much better hands with my fiancé and I. We own a house, both have full time, decent paying job, and can fulfill any necessity he will need. If anyone would could give me an advice, personal experiences, or even a thought. It's much appreciated. It's never my intention to take a child from their mother but I feel it's in the best interest of the child.