Psalm 133:1-3 — Behold, how good and how pleasant it is for brethren to dwell together in unity! It is like the precious ointment upon the head, that ran down upon the beard, even Aaron’s beard: that went down to the skirts of his garments; as the dew of Hermon, and as the dew that descended upon the mountains of Zion: for there the Lord commanded the blessing, even life for evermore.

Did you ever wonder why the Psalmist makes such a big deal,of this. Well, it’s not certain who wrote Psalm 133 — but if it was David, he certainly knew about bother with brothers.
It is a beautiful thing when brothers dwell together in unity. It’s a beautiful thing when sisters dwell together in unity to! The trouble is … It doesn’t happen nearly as often as it should.
Why is it, do you think, that brother so often fights with brother, and sister so often falls out with sister? Why did Cain kill Abel, why did Leah hate Rachel? I think I have an idea.
If you have brothers and sisters, did you happen to go down to the sibling store and pick them out? I didn’t think so. Children are God’s gift — “Lo, children are a heritage of the Lord : and the fruit of the womb is his reward. As arrows are in the hand of a mighty man; so are children of the youth.” (Psalm 127:3-4)
The other thing about families is that most of us (if we’re lucky, as I was most of the time) have to grow up with them — and when you have to live in close contact with a bunch of people that you didn’t pick there’s friction. No surprise really.
So it’s not so surprising that families aren’t always happy. How come brothers pick on brothers, and sisters scratch at sisters? Well, here’s a couple of key scriptures:

And the boys grew: and Esau was a cunning hunter, a man of the field; and Jacob was a plain man, dwelling in tents. And Isaac loved Esau, because he did eat of his venison: but Rebekah loved Jacob. (Genesis 25:27-28)
And Laban had two daughters: the name of the elder was Leah, and the name of the younger was Rachel. Leah was tender eyed; but Rachel was beautiful and well-favored. And Jacob loved Rachel; and said, I will serve thee seven years for Rachel thy younger daughter. (Genesis 29:16-18)

Did you get it? Brothers are different. Sisters are different. Those differences affect the way people react, and we notice. “You like him, better than you like me”, “She’s your favorite, it’s not fair”! I love the Bible. It just describes reality perfectly.
So ok, I think I understand it — but what’s to do? Let me know if you have the perfect answer. I know it’s possible — Mary and Martha lived (more or less) in harmony. Simon Peter and Andrew … James and John … They pulled it off. All I can think of is something that Rick Warren talked about. There are people you have to regard as “extra grace required” … and maybe that applies to brothers and sisters more than anybody else!