The feeling of rejection can be a common feeling among adoptees. The rejection can come from so many different places and at so many different degrees. Does the rejection affect an adoptee? Yes, any rejection affects people.
You most likely will feel rejected by your birth family at some point that does not mean you will always feel this way. Most adoptees work through these feelings on their own. The rejection for some adoptees goes much deeper and can cause them great pain.
Rejection by society can be everywhere an adoptee turns. The media seems to only talk about the negative aspects of adoptees. Schools can make a child feel different and rejected by their lessons on adoptees and adoption. Some school districts also label an adopted child at risk even in kindergarten or before they see signs of a child having problems in school. Just because a child is adopted does not mean they will have learning disabilities or struggle with school.
Family members can also reject an adoptee. Speaking from experience as a child, no matter how young, one knows and feels the rejection by family members even if others act like it does not exist. My older sister (birth child) always had issues with me from the beginning. To this day I get the ever so often reminder that I am adopted. When my mother was hospitalized, she stated she was all mother had and went on and on about if something should happen to mother, what she would do. This can have a great effect on an adoptee.
Sadly, there are some adoptees that are rejected by their adoptive parents. You seem to hear how the birth children were treated differently than the adoptee. This can be very devastating to an adoptee.
Adoptees that are adopted from other countries or inter racial adoptions can also feel rejected by home countries and their ethnicity. This can bring up a lot of feelings for an adoptee.