If my sis in law gives up rights to her mom for her son and gives her son to her does she still pay child support if so how long does she pay it for?

She pays it until the child reaches the age of majority based on the state you live in. Was the child adopted by the grandparent or was it just guardianship ?

What makes you think she can give up her rights? There is this myth that a parent can sign over their rights, but its just a myth. The only way she can get out of paying support is if someone adopts the child.

First it is almost impossible to give up your rights, this would mean that the mom is going to adopt the child, Merely raising the child, or being given guardianship, the person will pay support and do so, till the child is 18 or 21 years of age.

If the mother is going to adopt the child, the bio father also has to agree to the adoption, and the court will have to allow it also