Acts 1:14 They agreed they were in this for good, completely together in prayer, the women included. Also, Jesus’ mother, Mary, and his brothers. (The Message Bible)
In Luke 8, and remember Acts is the second volume of Luke’s work of which his gospel is the first, we find, “His mother and brothers showed up but couldn’t get through to him because of the crowd. He was given the message, ‘Your mother and brothers are standing outside wanting to see you’” (19, 20). We see that at this point Jesus’ family is on the outside looking in. Jesus’ response on that occasion was “My mother and brothers are the ones who hear and do God’s Word. Obedience is thicker than blood” (21). Now in the verse above we find them in the group of the first followers of Jesus after the first Easter. These are the folks who said that Jesus not only rose from the dead but that he was sinless and actually divine. How hard would it be to get family members, those who watched him grow up and knew him best, to come to such a conclusion? Something pretty dramatic, I would suppose. Maybe a resurrection?