1 Behold, how good and pleasant it is
when brothers dwell in unity!
2 It is like the precious oil on the head,
running down on the beard, on the beard of Aaron,
running down on the collar of his robes!
3 It is like the dew of Hermon,
which falls on the mountains of Zion!
For there the Lord has commanded the blessing,
life forevermore. Psalm 133 ESV
That the history of sibling rivalry begins as far back as Genesis probably doesn’t surprise any of us, brother and sister alike (Genesis 4). That the history of the restoration of all our relationships; with God, and our neighbour, as well as our family, is tied to the history of God’s love shown towards us, gives us ground for hope that whatever separation is caused by sin may be healed, and that here, as in all respects, our healing begins with the grace of God.
Derek Kidner commented on Psalm 133: “The second half of verse 3, with its strong accent on God’s initiative (commanded) and on what is only his to give (life for evermore), clinches another emphasis of the psalm, which is made by a threefold repetition, partly lost in translation: literally, ‘descending (2a) … descending (2b) … descending’ (3a). In short, true unity, like all good gifts, is from above; bestowed rather than contrived, a blessing far more than an achievement.”
From Derek Kidner in his Tyndale Old testament Commentary on Psalms