Many theoretical concepts are used in the practice of family therapy. One of the most common is family systems therapy. This is based on the theory that the family is the identified patient and examines the relationships and communication patterns within the family. A comprehensive look at this theory can be read at Serendip.com.
Out of family systems theory, grew an examination of the roles within families with addiction issues. In such dysfunctional families, children often take on a role, such as "scapegoat," "hero child," or "lost child." Scapegoat is fairly self-explanatory. The family, due to denial, is unable to address the real problem - the alcoholic or addict's use, so they blame another family member. This is usually the child who gets into trouble at school or with the law. The hero child is the one getting good grades, never in trouble, etc. The family can point to this child and say, "there's nothing wrong with THIS family!" The lost child is often the quiet one who sits in the corner playing with toys. Without help for the family, all these children are at risk for health and mental health issues.
Claudia Black, PhD, MSW, a pioneer author and lecturer on recovery and family addiction is an excellent resource. I highly recommend her workbook, "Repeat After Me," which has exercises to break dysfunctional family roles.
In a less clinical examination, most of us have a role in our family. Sometimes it's the older brother who always takes charge, the care-taking sister who "knows what's best" for everyone or the "baby," who just by virtue of being the youngest, gets all the attention. What about "slacker brother who can't keep a job" or "thrice-divorced sister who can't keep a man?" The latter is extreme, but the general idea is universal.
Choose to Step Out of Your Role
It might not be fair to label each other this way, but sometimes it's more comfortable for our family members to see us in these familiar roles. But that doesn't mean you have to assume or play that role forever. Your family may have an adjustment period, but they will have to adapt to your new role.
Healthy families can joke about their family roles. Being open and direct about your role and expressing your feelings can help your family have a better understanding of how dysfunctional it can be. One of my favorite movies is a comedic examination of family roles - "Home for the Holidays" Rent or borrow it this season.