How can you find a sibling that was adopted at birth?

First your sister has to be over 18 before you ever try to contact her. Birth families are just not allowed to contact the adopted child until they are adults. You need to gather as much info as you can, place of birth, if she was named by your mother, address to the adoption agency, names on the original birth certificate mother and father. There is a lot of research involved before you can even begin your search. Once you have all of the info you should first contact the adoption agency and see if they will release any information about the adoption. Even if they release very little it is better than nothing. You may find out the names of the adoptive parents or at least where they lived at the time of the adoption. You can also go on line, there are a few web sites that are devoted to finding adopted people and reuniting them with birth families. It is a long shot, but sometimes when someone finds out that they are adopted they want to find out about their birth parents. Just remember, she may not know she is adopted, or if she knows she may not care to meet her birth family. She has grown up away from you and may not want that connection to a family that put her up for adoption.