I was just reading another thread here about the Song of Solomon. I haven't read it for a while but when I did I noticed the groom repeatedly referred to his bride as his sister and in relatively (for the time period) explicit sexual ways.
Ravi, this speaks of the spiritual relationship with Jesus Christ who is the Bridegroom in Solomon's Song. ( The Believers - who are the church - the body of Christ - are His Bride)
When He says, Open to me my sister, my love, my dove, my perfect one...... in Solomon's Song 5: 2 - He uses the word sister in reference to a spiritual relationship - He is the firstborn among many brethren. When he says, my love - he is speaking to the object of His heart's devotion, when he uses the words, my dove - He is speaking to the one who has been endued with many gifts and graces of the Holy Spirit - the Holy Spirit is represented by the dove, when he speaks calling her my perfect one - He is speaking of the born again Believer who is washed, renewed and cleansed by His precious Blood at the cross.
My husband, who married me, is my brother in Christ as he is a born again Christian. Is he blood related to me? No. I hope that answers your question and part 5 and 6 will be coming up later so keep reading the thread for the conclusion of the Book.