As so often, this proverb will appear to state a strange and ill-advised sentiment if we do not make certain assumptions in interpreting it. Solomon is expressing truth in short compass, and he therefore abbreviates his statements and expects the reader to supply the unstated background that is necessary to give a good sense.
When we enter into God’s kingdom our allegiances change; what we have in common with others is no longer the same. Our unconverted family members no longer know us as they did, for ‘a prophet is not without honour, save in his own country, and in his own house’ (Matthew 13:57). We turn to those who understand us, but who can understand a believer except another believer. When Christ’s own unconverted brothers tried to advise him, they missed the mark completely: ‘Depart hence, and go into Judaea, that thy disciples also may see the works that thou doest. For there is no man that doeth any thing in secret, and he himself seeketh to be known openly. If thou do these things, shew thyself to the world’ (John 7:3-4). Therefore, better a neighbour, a converted neighbour, than a brother who is far away from the Lord and from us.