Marrying a Sibling?

I'd came across to a post in a social media group with this question - 

Why do Mormons marry their siblings? 😳

A post from a Random Group
of Christian and Mormon Discussions. 

Found this post from one of the groups that I'd been and this one has something that needs to be understood.

I have been in a church for my entire life and I never heard such teaching about marrying a Sibling. But yes, we address everyone as brothers and sisters. The scripture had said that we are God's children his offspring. And if you take it literally, it's clearly understood that we are Brothers and Sisters.

I don't understand how the teaching biblically was applied to each Christian. I am not sure what he meant to be Siblings (literally? I don't know) but something is clear, every one of us are God's children of the same species spiritually.

So, my questions are - 

• Who are we going to marry?
• If he or she is not the son or daughter of God, who's son/daughter is he/she? 
• Do Christian world understood the same passage of Scripture and accept it as God's word?
• If so, then there's no question about marrying God's children, since we all are God's Children?

The scripture was given for us to know the mystery of the kingdom of God and the principles we need to practice to earn salvation. And as the scripture taught us that God is our Father in heaven no further revelations need to be revealed but to accept it as his words.

Here are some of the reference of this topic - 

[ Revelation 1:9] - I John, who also am your brother, and companion in tribulation, and in the kingdom and patience of Jesus Christ, was in the isle that is called Patmos, for the word of God, and for the testimony of Jesus Christ.

John address himself as our brother and companion...

[Acts 17:27-29] 

27 That they should seek the Lord, if haply they might feel after him, and find him, though he be not far from every one of us: 

28 For in him we live, and move, and have our being; as certain also of your own poets have said, For we are also his offspring. 

29 Forasmuch then as we are the offspring of God, we ought not to think that the Godhead is like unto gold, or silver, or stone, graven by art and man’s device.

We are God's Offspring. Was it mean to translated literally or something else?

[Psalm 82:1,6-7] God standeth in the congregation of the mighty; he judgeth among the gods. 

... 

6 I have said, Ye are gods; and all of you are children of the most High. 

7 But ye shall die like men, and fall like one of the princes.

We are gods, which means we inherit a characteristic just like our Father and has that same potential to be like him.

This certain doctrine will just simply conclude that God is not a different species as we are, and that we are created in the image of God and its likeness having the potential to be like him.