[HarleyD]

If we consider for a moment that "brethren" meant "cousin" what our Lord Jesus' would be saying is:

Who is my mother or cousins? Those who do the will of the Father. They are my brothers (or substitute cousins) and sisters and mother.

In either case this makes little sense at all in my opinion.

[dangus]

So you are trying to assert a parallelism between "My mother and My brethren," and "My Brethren and sisters and mother." The reversal in the word order, plus the inclusion of "sisters" makes it hard for me to believe that such a paralellism was intended. If there is a paralellism intended, Jesus is purposely becoming more specific in response by referring to his brother and sister, only emphasizing the point that the earlier use of the word "brothers" did not specifically refer to male, nuclea-family relatives.