The reading from today’s gospel (with a few extra verses) and St. Francis of Assisi’s answer to the question.
20 Jesus came with his disciples into the house. Again the crowd gathered, making it impossible for them even to eat. 21 When his relatives heard of this they set out to seize him, for they said, “He is out of his mind.” … 31 His mother and his brothers arrived. Standing outside they sent word to him and called him. 32 crowd seated around him told him, “Your mother and your brothers* [and your sisters] are outside asking for you.” 33 But he said to them in reply, “Who are my mother and [my] brothers?” 34 And looking around at those seated in the circle he said, “Here are my mother and my brothers. 35[For] whoever does the will of God is my brother and sister and mother.” (Mark 3)
St. Francis of Assisi: “They are children of the heavenly Father whose works they do, and they are spouses, brothers, and mothers of our Lord Jesus Christ. We are spouses when the faithful soul is joined to our Lord Jesus Christ by the Holy Spirit. We are brothers to Him when we do the will of the Father Who is in Heaven. We are mothers when we carry Him in our heart and body through divine love and pure and sincere conscience and give birth to Him through a holy activity which must shine as an example before others.”
Earlier Exhortation, para 8 (also known as Second Letter to the Faithful)
Francis of Assisi: Early Documents, Vol. 1: The Saint, p.42
The Franciscan Institute of St. Bonaventure University, 1999
Eds.: Regis J, Armstrong, J. Wayne Hellman, William J. Short.