I started paying attention to this comment about Jesus being the brother of Satan only in the last few years here in the US. I became a member of the Church of Jesus Christ of Latter-day Saints (Mormon Church) in Italy and I had never heard this argument when people tried to attack the Mormon Church.

While living in Brazil I heard it only a few times, but I thought that it could be dismissed very easily, since it seems to me a very dumb comment anyway.

Since we are all children of God, brothers and sisters in spirit, then Jesus is the brother of Satan the same way some of us have brothers and sisters, but this fact doesn’t really tell us anything about what we believe, or it does not mean that the Mormon Jesus is someway “friend” with Satan or that they stand for the same things

However, during the General Conference of the Church of Jesus Christ of Latter-day Saints I began noticing people who were raising signs like the one below.

Clearly, some people extrapolate all sort of meanings from the simple belief (and fact) that Jesus and Satan (and all of us) were spiritual brothers and sisters before this life.

I think that a recent article from MDL.org helps to understand how this belief is twisted to attack Mormon beliefs.

Many anti-Mormons claim that Mormons are not Christian. But Mormons believe in Jesus Christ and in His Atonement for our sins. It is only through Jesus that we are saved. Many Christians who claim you only have to accept Jesus to be saved have a problem with Mormons because we likewise accept Jesus as our Savior and Redeemer, despite our doctrinal differences. So, the thought goes among some who refuse to accept us as Christians, that we must believe in a different Jesus, not the Jesus of the Bible. As supposed proof, these critics argue that the so-called “Mormon Jesus” is the brother of Satan, and therefore it doesn’t matter if Mormons accept him or not because he is the wrong Jesus. Alternatively, to other detractors, accepting the “Mormon Jesus” actually matters very much, because following the “Mormon Jesus” would be the same as following Satan, his “brother.”
Whichever the case, this ridiculous notion is a “straw man” or a gross misrepresentation of actual LDS belief…In short, the entire argument is a disingenuous attempt to demean and belittle Mormons. It is bigotry pure and simple.