adopted male born 12/12/40 Evansville, IN iso sibling (male?) born 2-3 years later

My father was born to a teenage mother in Evansville, Indiana on or about 12/12/40. His birth father info is unknown. He was adopted privately through a Catholic church. His birth mother had a second child around 1942--1943 who may not have been adopted. We think it was a boy. My parents went to Indiana years ago to search for this info and returned home to Nevada empty handed. My father is still alive and will be 71 this December. Thank you for taking the time to read this.