Growing up in a family directs our behavior in a different way than it would be directed in an orphanage or group foster home. Children are to be sent to school and not beaten. Girls do the dishes and boys take out the garbage. The thing to remember is that social institutions direct our behavior, but not absolutely. There is some wriggle-room.
Families can choose to have boys do dishes and girls take out garbage.
Essentially, agency means working against social structure in a way that meets our needs. For instance, when you entered Aquinas there was already a social structure that defined the roles and behaviors of students, faculty, and administrators. In terms of family, the nuclear family structure is, in a way, a given for us because it is so frequently touted as the Norm. Again, these are built into the social structure. Biologists will tell you there are no pure races.