The whole section [including the Parable of the Seed] until this point has given emphasis on the Word. From Luke 8:11 until this section, there are five occurences of "word" (vv. 11,12,13,15,21]. In these occurrences "word" is always an object of the verbs "accept/receive" "hear" and "put (it) into practice" (v.21).
The account of Jesus' mother and relatives wanting to see him is found also in Mark and Matthew. But in Luke, the reference to what the mother and brothers think of Jesus is omitted. Mark writes that these thought he was out of himself. Luke doesn't include that detail here. The intent is to emphasize how the Lord looks at those who habitually (the verbs are in the present participle, akouonteV and poiounteV) listening and doing his word.
My mother and brothers are those who hear God's word and put it into practice. (Luke 8:19-21)
Some think that by this statement the Lord is denying any importance to his blood relatives, especially his relationship to his mother Mary. They fail to notice that it is in the Gospel of Luke that special emphasis is placed on Mary as the one whose acceptance of the Word proclaimed by the angel occassioned the birth of "the Son of the Most High." They also fail to mention that it is only in Luke where we find the Baptist described as a relative of the Lord, being the son of Mary's cousin Elizabeth.
Albert Nolan, OP has pointed out in his "Jesus Before Christianity" that far from declaring blood relationships null and void, the Lord has, by the declaration in v. 21, given a new dimension to the idea of "family." Thenceforward, anyone can already speak of the family of the Lord where the familial bond is constituted by fidelity to the Word of God.
The idea in Luke 8:21 is further developed in John 15 in the key idea "Remain in the Lord." Here, "to remain in the Lord" is "to remain in his love" and "letting his word remain in (you)" in a relationship that is described in terms of biological symbiosis.
When we think of the Church as the Family of Christ, we are in a sense, actually echoing Luke 8:19-21. The word "Church" itself derives from the adjective kuriakh which means "belonging to the Lord." But it is Family not in the sense that we can be "familiar" with the Lord, but in the sense that all those who consider themselves part of it are habitually hearing and doing the Word of God.
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Don't curse the darkness, light a fire. Don't wait for the sunrise. Walk towards the dawn.
