Sure a sister can make a mistake. What you need to remember is that she was a representative of her sorority and her actions reflect on the sisters and sisterhood of that sorority. This rule is also applied to teachers, police officers, lawyers, nurses judges, elected officials and so on. When a member of an organization, occupational group, or religious group make a mistake it will reflect on that person and the group they are associated. It is what it is. It's called consequences.