Is it considered blackmail (illegal) if I:

Threaten to tell someone's family that their aunt/mother adopted out a child when they were 15 unless said person's sister gives me information regarding medical history and details of the father of the child. There are no monetary demands.
The child adopted out is related to me, and they have had requests for medical information and details of their biological father refused to them by their bioloical mother.
Can I demand the information is given to us, and threaten to tell the biological mother's family about the adopted out child. The family currently have no knowledge of this adoption.

Will I be committing a crime?