Will court rule for the best interest of the child or will my brother be fucked because he's not the biological father?

My brother has had full custody of my nephew since he was one. His wife left and abandoned them both. She's been in and out of jail. She's 20k behind on child support. She's overall a shit person and didn't contact her son at all until she came back several months ago to tell my brother she wanted to take him back. He's 12 now. He has not seen his mother in 11 years. When my brother told her no shw confessed that he's not the father anyway. They had to go to court for a DNA test and my brother is not the father. That crushed him. Crushed my nephew. She was so happy in court but she's an idiot who didn't realize just because he's not the father doesn't automatically grant her custody. They still have to go to more court dates. My brother got a lawyer and theyve been going back amd forth to court. So far all she's been granted is visitation and shes missed the last 2. My brother is just worried and I'm curious if anyone has any experience in family law, will my nephews besy interest trump biology? He's been allowed to speak his wants in court to and has said he wants to stay with his dad.