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Parents have a constitutionally protected right to raise their children, which is not compromised by being too young. Sounds like your sister is young. She may not be able to re-establish her relationship with her child without involving the Court.
Her child may need time to reintigrate into her life. This may need to be handled with a therapist if the grandparents have alienated the mother from her children.
A court is always going to do what is in the child's best interest. The child's best interest is balanced against the parent's interests. Parents *can* theoretically be unfit - because they have drug problems, because they have mental health issues. It happens. But what you are describing sounds more like the grandparents decided (without benefit of court oversight) that she was unable to care for her child, and demanded she hand him over and now refuse to allow contact.
Chances are the grandparents have more money and can hire counsel easier. What your sister needs is a lawyer. If she can't afford one, she can contact her county's Bar association and see if they have a clinic or a low income access to justice program where she can get help and advice.
Your sister should not be taking legal advice from her parents, she should get her own attorney to present her side of the story so she can re-establish her relationship with her child free of undue interference from her parents. Hope this helps. XXXXX XXXXX