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‘My brother's keeper’ comes from Cain’s reply to God on being asked where his brother was.
Genesis 4:9 And the LORD said unto Cain, Where is Abel thy brother? And he said, I know not: Am I my brother‘s keeper?
The Hebrew word ‘shamar’ carries the meaning ‘to watch over’, ‘to preserve’.
The answer of course is that God expects us to look after our brother. Brother is not restricted to siblings but extends to our fellow man (and woman) for we are all related as descendants of Adam.
The Old Testament teaching is clear:
Genesis 9:5 And surely your blood of your lives will I require; at the hand of every beast will I require it, and at the hand of man; at the hand of every man’s brother will I require the life of man.
Leviticus 19:17 Thou shalt not hate thy brother in thine heart: thou shalt in any wise rebuke thy neighbour, and not suffer sin upon him. 18 Thou shalt not avenge, nor bear any grudge against the children of thy people, but thou shalt love thy neighbour as thyself: I am the LORD.
Leviticus 25:35 And if thy brother be waxen poor, and fallen in decay with thee; then thou shalt relieve him: yea, though he be a stranger, or a sojourner; that he may live with thee. 36 Take thou no usury of him, or increase: but fear thy God; that thy brother may live with thee.
Deuteronomy 15:7 If there be among you a poor man of one of thy brethren within any of thy gates in thy land which the LORD thy God giveth thee, thou shalt not harden thine heart, nor shut thine hand from thy poor brother: 8 But thou shalt open thine hand wide unto him, and shalt surely lend him sufficient for his need, in that which he wanteth.
Deuteronomy 22:1 Thou shalt not see thy brother’s ox or his sheep go astray, and hide thyself from them: thou shalt in any case bring them again unto thy brother. 2 And if thy brother be not nigh unto thee, or if thou know him not, then thou shalt bring it unto thine own house, and it shall be with thee until thy brother seek after it, and thou shalt restore it to him again. 3 In like manner shalt thou do with his ass; and so shalt thou do with his raiment; and with all lost thing of thy brother’s, which he hath lost, and thou hast found, shalt thou do likewise: thou mayest not hide thyself. 4 Thou shalt not see thy brother’s ass or his ox fall down by the way, and hide thyself from them: thou shalt surely help him to lift them up again.
Exodus 23:4 If thou meet thine enemy’s ox or his ass going astray, thou shalt surely bring it back to him again.
The New Testament teaching is equally clear:
Mark 12:31 And the second is like, namely this, Thou shalt love thy neighbour as thyself. There is none other commandment greater than these.
Matthew 5:44 But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;
1 John 3:15 Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him. 16 Hereby perceive we the love of God, because he laid down his life for us: and we ought to lay down our lives for the brethren. 17 But whoso hath this world’s good, and seeth his brother have need, and shutteth up his bowels of compassion from him, how dwelleth the love of God in him?
1 Corinthians 10:24 Let no man seek his own, but every man another’s wealth.
Philippians 2:4 Look not every man on his own things, but every man also on the things of others.
Romans 13:9 For this, Thou shalt not commit adultery, Thou shalt not kill, Thou shalt not steal, Thou shalt not bear false witness, Thou shalt not covet; and if there be any other commandment, it is briefly comprehended in this saying, namely, Thou shalt love thy neighbour as thyself.
Galatians 5:14 For all the law is fulfilled in one word, even in this; Thou shalt love thy neighbour as thyself.
Abram (Abraham) looked after Lot.
Genesis 14:14 And when Abram heard that his brother was taken captive, he armed his trained servants, born in his own house, three hundred and eighteen, and pursued them unto Dan... 16 And he brought back all the goods, and also brought again his brother Lot, and his goods, and the women also, and the people.
Joseph’s brethren failed to look after Joseph.
Genesis 37:28 Then there passed by Midianites merchantmen; and they drew and lifted up Joseph out of the pit, and sold Joseph to the Ishmeelites for twenty pieces of silver:
Joseph was of a different mind altogether, he bore no grudge and looked after his brethren.
Genesis 50:19 And Joseph said unto them, Fear not: for am I in the place of God? 20 But as for you, ye thought evil against me; but God meant it unto good, to bring to pass, as it is this day, to save much people alive. 21 Now therefore fear ye not: I will nourish you, and your little ones. And he comforted them, and spake kindly unto them.
Moses looked after Aaron.
Deuteronomy 9:20 And the LORD was very angry with Aaron to have destroyed him: and I prayed for Aaron also the same time.
Moses looked after Miriam.
Numbers 12:13 And Moses cried unto the LORD, saying, Heal her now, O God, I beseech thee.
Moses looked after Israel.
Numbers 14:12 I will smite them with the pestilence, and disinherit them, and will make of thee a greater nation and mightier than they. 13 And Moses said unto the LORD... 19 Pardon, I beseech thee, the iniquity of this people according unto the greatness of thy mercy, and as thou hast forgiven this people, from Egypt even until now. 20 And the LORD said, I have pardoned according to thy word:
Ruth looked after her mother-in-law.
Ruth 1:16 And Ruth said, Intreat me not to leave thee, or to return from following after thee: for whither thou goest, I will go; and where thou lodgest, I will lodge: thy people shall be my people, and thy God my God: 17 Where thou diest, will I die, and there will I be buried: the LORD do so to me, and more also, if ought but death part thee and me.
Ruth 2:11 And Boaz answered and said unto her, It hath fully been shewed me, all that thou hast done unto thy mother in law since the death of thine husband: and how thou hast left thy father and thy mother, and the land of thy nativity, and art come unto a people which thou knewest not heretofore. 12 The LORD recompense thy work, and a full reward be given thee of the LORD God of Israel, under whose wings thou art come to trust.
David looked after Jonathan’s son.
2 Samuel 9:1 And David said, Is there yet any that is left of the house of Saul, that I may shew him kindness for Jonathan’s sake?.. 6 Now when Mephibosheth, the son of Jonathan, the son of Saul, was come unto David, he fell on his face, and did reverence. And David said, Mephibosheth. And he answered, Behold thy servant! 7 And David said unto him, Fear not: for I will surely shew thee kindness for Jonathan thy father’s sake, and will restore thee all the land of Saul thy father; and thou shalt eat bread at my table continually.
Jehoiada looked after Joash, Ahaziah’s son and restored him to the throne.
2 Chronicles 22:11 But Jehoshabeath, the daughter of the king, took the son of Ahaziah, and stole him from among the king’s sons that were slain, and put him and his nurse in a bedchamber. So Jehoshabeath, the daughter of king Jehoram, the wife of Jehoiada the priest, (for she was the sister of Ahaziah,) hid him from Athaliah, so that she slew him not.
Joash failed to look after Zechariah, Jehoiada’s son but had him killed.
2 Chronicles 24:22 Thus Joash the king remembered not the kindness which Jehoiada his father had done to him, but slew his son. And when he died, he said, The LORD look upon it, and require it.
In the New Testament the parable of the Good Samaritan (Luke 10:29-37) shows us that all in need are our neighbours.
Jesus of course is the ultimate example.
Romans 5:7 For scarcely for a righteous man will one die: yet peradventure for a good man some would even dare to die. 8 But God commendeth his love toward us, in that, while we were yet sinners, Christ died for us.
I hope you have found this helpful.