Do Protestants believe Jesus and Satan are brothers? Many, many times, I've seen or heard &quot;Mormons believe Jesus and Satan are brothers&quot;.
Technically, it's true. Mormons believe Jesus and Satan are sons of God, and by dictionary definition, people can extend this to say &quot;brothers&quot;. (Jesus's is also God's son in the flesh as well as in the spirit, but nonetheless, the statement holds true.)
For example

One answer to what is different about Jesus in the LDS faith is that Jesus and Satan are brothers.

There is According to LDS (Mormon) teaching, what is the relationship between Jesus and Lucifer?, which boils down to &quot;spirit brothers&quot;.

And literally hundreds more instances as seen with a simple Google search.


My surprise, I suppose, is that this is a surprise. I had thought other Christians believed the same thing, technically.
Possibly relevent scriptures:
God has sons (angels?)

Where wast thou when I laid the foundations of the earth? declare, if thou hast understanding.
Who hath laid the measures thereof, if thou knowest? or who hath stretched the line upon it?
Whereupon are the foundations thereof fastened? or who laid the corner stone thereof;
When the morning stars sang together, and all the sons of God shouted for joy?
(Job 38:4–7 (KJV))

And Satan is a fallen angel... a (disowned) son of God?

How art thou fallen from heaven, O Lucifer, son of the morning! (Isaiah 14:12)

And Jesus is a son of god:

I ascend unto my Father, and your Father; and to my God, and your God. (John 20:17)

Does mainstream Protestantism believe that Jesus and the devil are &quot;brothers&quot;? If so, does the belief differ from Mormonism in any way? If not, what is the belief and how does it differ?
Good question with a simple answer: No. Categorically no.

All mainline Protestants (and actually most branches of Christianity including Catholic and Orthodox) believe that they are radically different. "Categorically" different if I may overload that word.


Jesus is God.

Son of God in that he is the "son" part of the Trinity, but the very person of God, not a separate being.
Satan is a created being.

As an angel or archangel or whatever (the nuances of what sort of creature he is vary between traditions) he is in every way separate from and subordinate to God. There is no relation or lineage, and he is not himself a God.


Furthermore Protestants (and again most Christian groups) believe that God's work of creation was done through the person of Jesus per Colossians and others:


  The Preeminence of Christ
  
  He is the image of the invisible God, the firstborn of all creation. For by him all things were created, in heaven and on earth, visible and invisible, whether thrones or dominions or rulers or authorities—all things were created through him and for him. And he is before all things, and in him all things hold together.
  
  (Colossians 1:15-17 ESV)


Satan would fall in the one of the categories above (e.g. "thrones" or "dominions"). In other words: Jesus created Satan. They are not on a level.