This chapter is a collection of parables. Parables are short stories or vignettes that have a central point. A parable, like a painting, should first be examined at a distance so the whole picture can be seen. Unlike the parable of the sower (13:1-9 and 18-23), most parables do not necessarily have a one to one correlation. This is the one parable that Jesus takes the time to explain to his followers (along with the wheat and the weeds later in this chapter), all of the others are left to instruct, to challenge and, ideally, to let us catch glimpses of the kingdom Jesus is ushering in. Jesus also explains, in 10-17, the purpose of speaking in parables. The point is pretty simple, Until one chooses to live a Christian life one cannot understand what Jesus was getting on about. Those who come to know and follow Jesus have an experiential understanding of kingdom life.
The parable of the wheat and the weeds, the mustard seed, a hidden treasure, a fine pearl, and the broad cast net show fundamental truths of the Kingdom of the heavens: from small beginnings (the mustard seed) come great conclusions (a tree); it is worth all we have to live into it (treasure and pearl); there is room for everyone -- but not everyone will get it.
51-53 is an echo of the wine skins illustration that Jesus uses earlier in the gospel. When the new is breaking in we tend to stick with what is comfortable and familiar.
Notice in 54 that Jesus brothers are named James, Joseph and Simon and Judas. These, by all appearances, are Mary and Joseph's younger children (Jesus being the first born). James and Jude are identified as leaders in the early Christian movement. This is the James who is referred to as "the Lord's half brother" and is presiding at the Jerusalem Council in Acts 15 and is generally believed to be the author of the Epistle of James. In 1 Corinthians 15 we learn that James was accorded a special post resurrection from Jesus. Judas (Jude) is generally believed to be the author of the Epistle of Jude. Following Jesus death and resurrection his brothers and his mother become an integral part of the early Christian movement.
Some denominations argue that the word for brothers can also mean cousin and that these are simply kin folks of Jesus'. Those that argue this usually have a theological presupposition of the perpetual virginity of Mary. Since she was always a virgin, they argue, she and Joseph could not have produced children. It is an excellent example of a theological idea requiring a reinterpretation of the biblical text. Given life in the 1st century, there is little to suggest that Mary and Joseph would not settle down to a fairly normal life and that part of that life would be the conception and raising of other children. In some ways this humanizes the "holy" family and makes them more accessible to our life and understanding.