Custody Battle

My little sister and her wife have a 4 year old boy. My sisters wife had him when she was 16, the baby daddy isn’t on the birth certificate. He’s always claimed it wasn’t his. He just decided that he wanted to play daddy all of a sudden & wanted her to start dropping him off at his house. he doesn’t pay child support, she was told he wouldn’t be able to because he isn’t 18 yet. she just received a message from the boys dad

it’s a petition to grant the baby daddy full custody and to change my nephews last name to his. Any advice on how to move forward? will she need a lawyer? have any of you gone through this? he’s lived with her his whole life, she pays for everything with the help of my sister now of course but is it likely the judge will approve this?

the grandparents used to be able to see him on the weekends when THEY had the time but they were never willing to meet her half way, she used to drop him off & pick him up but when the pandemic hit she wasn’t as willing anymore because the baby was always sick & she didn’t know who would be coming around him.