Sometimes, adoptions result in the separation of birth siblings. This may be something that happens later in life, for example siblings in foster care may be split up. Other siblings may have been separated by adoption years before the other was born or at the time a younger sibling was born.
The person who chooses to search may be a sibling who grew up with the adoptee's birth parents. Maybe they're trying to reconcile one of their last familial ties after the birth parents have passed. Or maybe they are curious about a hereditary disease they and the adoptee may share. Regardless of the reason to search, the results can sometimes be shocking and less satisfactory than one's ideals. It's highly suggested that anyone who decides to search for a birth relative first joins a support group for searching adoptees and birth relatives.
As for the actual process, searchers should register with their state's registry. Then, they may want to contact the agency they were adopted from. International sibling searches are going to be more complex and there are certain organizations that specialize in specific country searches for adoptees. If the adoption was of a domestic infant, then an children the adoptee's birth parents had prior to the adoption would be listed in the sealed adoption record. It may indicate basic information about the child, such as his or her age and gender.