Saying that Mormons believe that Jesus and Satan are brothers does not misrepresent your position, after all according to LDS teachings Jesus and Satan are brothers. It does however should with absolute clarity how far apart the LDS are from Christian belief.
Being presented with the difference in a manner many (if not most Christians) see as blasphemous hurts proselytizing efforts with Christians, that’s why LDS members object so strenuously.when others legitimately point it out.
Jesus the brother of Satan and Jesus the creator of Satan take you to two different discussions, the first about the nature of God the second about the problem of evil, it is a perfectly valid to point out that LDS believe Jesus is the brother of Satan, because it leads directly to a discussion of God’s nature. LDS seem to be afraid of this conversation when they cry foul over the Jesus, Satan brothers thing.