This is probably one of the most amazing text critical decisions I have seen in an quite a long time. I checked the ESV against the ASV, NASB95, KJV, RSV, NRSV, TNIV, NIV, HCSB, and NLT on this one. The ESV follow the RSV alone in this one. Its really striking:

46 While he was still speaking to the people, behold, his mother and his brothers stood outside, asking to speak to him. 48 But he replied to the man who told him, “Who is my mother, and who are my brothers?”

Note: Some manuscripts insert verse 47: Someone told him, “Your mother and your brothers are standing outside, asking to speak to you”

Matthew 12:46-48.

I don’t think following the RSV here was the best course of action. The verse may be lacking in א^*B and a few others, but it seems to be too clearly of an error of the same kind we see P46 at Ephesians 1:3 where the first half the verse is missing. Both Matt 12:47 and Ephesians 1:3 definitely falls under the heading of  exceptions where the preference for the shorter reading should be rejected. I’ve sent my suggestion for the verse’s addition to the ESV committee.