Have your sister-in-law give her contact information to the adoption agency that handled the adoption. Also she needs give her written permission to give out her information to her son. If the adoption agency has closed, contact the agency that is handling the records. I would assume a state agency is probably involved.
I know a friend who was adopted from The Tennessee Children's Adoption Agency about 60+years ago. Due to some illegal activity by the director and a judge, the state opened the adoption records to the children that were adopted during that period of time. My friend's brother was able to find his biological mother and half sister. They could not find anyone in her family, but through the records she received, she was able to research the genealogy of her birth parents. In researching the genealogy, she found a living cousin on one parent's side who was open to talking to her.