Scripture says a lot about the relationship with or responsibility of parents to children and children to parents.  Much instruction exists for the husband and wife relationship.  The Bible says less about siblings.  Overall, the Lord Jesus Christ taught something to siblings in Matthew 12:

46 While he yet talked to the people, behold, his mother and his brethren stood without, desiring to speak with him. 47 Then one said unto him, Behold, thy mother and thy brethren stand without, desiring to speak with thee. 48 But he answered and said unto him that told him, Who is my mother? and who are my brethren? 49 And he stretched forth his hand toward his disciples, and said, Behold my mother and my brethren! 50 For whosoever shall do the will of my Father which is in heaven, the same is my brother, and sister, and mother.

That is akin to Jesus’ teaching in Matthew 19:29:

And every one that hath forsaken houses, or brethren, or sisters, or father, or mother, or wife, or children, or lands, for my name’s sake, shall receive an hundredfold, and shall inherit everlasting life.

Brothers and sisters should meditate on this, when they put their so-called relationships ahead of the Lord Jesus Christ.  Their relationship is headed for disaster when they do so — the worst possible disaster.

I don’t see anything unique about a sister and sister relationship in God’s Word that is any different than a relationship with anyone else.  They feel a natural commitment to one another, which I understand through observation and reading.  It must be natural or else Proverbs 7:4 wouldn’t say:

Say unto wisdom, Thou art my sister; and call understanding thy kinswoman.

That’s about it though, besides several verses in the Old Testament that prohibit sisters from seeing each other in their nakedness.

As little as there is about sisters and sisters, there is little to nothing about brothers with brothers.  Like the sister, brothers have a natural closeness.  Proverbs 18:24 reads:

A man that hath friends must shew himself friendly: and there is a friend that sticketh closer than a brother.

I have a brother, and I understand this.  Brothers might fight with each other, especially as seen with Cain and Abel.  Mine was a little brother (although he became bigger than me), and we fought, but if someone threatened him, I was there for him.  I still am.  I want the absolute best for my brother.  What we do have is the example of Jesus with His brothers, and what did He do with them?

In the beginning of John 7, we see Jesus with his brothers.  They weren’t aligned with each other in the truth.  They didn’t believe in Him.  He told them the truth in verses 6-7:

My time is not yet come: but your time is alway ready.  The world cannot hate you; but me it hateth, because I testify of it, that the works thereof are evil.

The world could not hate them, because they were compatible with the world.  The world hated Him.  Jesus told them the truth, a general no-no, it seems, for families with different beliefs today.  It’s what Jesus did though, whom we are to follow.  He didn’t try to get along with them — again, told them the truth, as a basis for whatever unity they would have.

However, the Bible has instruction for brothers and their sisters, all of it directed toward the brothers.  Brothers have a responsibility to their sisters.  When one considers everything being said in the Bible about this subject, it really is one responsibility of a brother to a sister:  protection — and in particular protection of her purity.  A brother isn’t there to entertain his sister, he’s not there to give her a fun time, and he is not there for her to cheer him up or validate him.  We can point to Miriam saving her very, very little brother Moses, but that isn’t laid out as a responsibility for a sister with her brother.

The brother to the sister shows up all over the Bible and all the same type of lesson.  I’m going to start with Song of Solomon 8.  In the context, the virginity of the Shulamite woman, Solomon’s potential mate, is compared to a beautiful garden.  She is to protect it like disallowing a garden to be trampled by defrauders, but not just her.  Her brothers are also responsible by what we read in verses 8-9:

8 We have a little sister, and she hath no breasts: what shall we do for our sister in the day when she shall be spoken for? 9 If she be a wall, we will build upon her a palace of silver: and if she be a door, we will inclose her with boards of cedar.

They will be happy if she will take responsibility, but they also will be a part of it, by enclosing her with boards of cedar.  Are you doing this brother?  Are you allowing laxity to your sister?  Are you a window or a screen door, not paying attention to the ways your sister is being or might be defrauded?  Are you helping her spiritually to be submissive to her father (1 Corinthians 7:36-38), because that’s what you would do if you were obedient to God?  What I’m writing is that there isn’t just the physical protection, but the spiritual guidance that a real man, a biblical, godly man would provide.

This job of protection of a sister by brothers or a brother is seen in the example of Laban with Rebekah in Genesis 24:29-30:

And Rebekah had a brother, and his name was Laban: and Laban ran out unto the man, unto the well.  And it came to pass, when he saw the earring and bracelets upon his sister’s hands, and when he heard the words of Rebekah his sister, saying, Thus spake the man unto me; that he came unto the man; and, behold, he stood by the camels at the well.

Later she got permission from the brother to go and marry Isaac.  Of course, there was no father alive there, but this indicates the responsibility of the brother.  The worst example of this is Amnon with his sister Tamar in 2 Samuel 13.  I’m not going to get into the details of the story, but the brothers did not protect Tamar in the worst possible way.  All brothers should make sure that the reputation of their sisters are kept pure in the most complete way.  At one time in our culture, this meant a lot.  If that’s not happening, that correction should be made, because in scripture that is the most important task of the brother.