Searching for Birth Sisters

I am searching for two sisters born in Pullman, Washington to Paul and Donna Gravelle. Colleen Ann Gravelle was born January 31, 1971 and Brenda Marie Gravelle was born March 16, 1974. They are the two youngest in a family total of 8 siblings all placed into adoption-----6 before they were born. Social Services was probably involved in the removal of my sisters. At the time the birth mother absent from the home, and birthfather worked in farming or laborer at a chicken ranch. Girls were suppose to have been adopted by families in Tacoma, Washington area who knew each other. I am the oldest sibling.