I guess this is why the Nicene Creed says that Jesus "was begotten not made." If Jesus was made, then there is the possibility that God could make other sons.
To hold that Jesus was made in the same sense that Lucifer was made can thus be safely at least declared "non-orthodox" and out of keeping with orthodox Christianity, at least as it has been defined since 325 AD.
A Mormon, of course, will object that the Nicene Creed wasn't given by revelation. That may be true, but it still puts them at odds with 1700 years of Christian scholarship, and so in my mind, at least raises the bar / puts the ball in their court to prove otherwise.
Ask: Who made Lucifer
Let's digress for a second and ask who made Lucifer.
As John 1:3 reminds us, there was nothing made that Jesus did not make. Thus, the question is "Is Lucifer a created being?" If the answer to that question is yes, then Jesus (via 1:14) made Satan and not God. As such, John 1:3 must be wrong or they are.
If Jesus made Satan, Jesus is Satan's father - not his brother.
God would at best be his grandfather, although as stated above, orthodox Christianity states Jesus was begotten not made. In any event, Jesus and Satan can't both have been made by God, since all things that were made were made by Jesus.
If they say 'No, Lucifer was not a created being," then ask them how Satan is not co-equal to God. If they assert that he is, they are Gnostics, as evidenced by their dualism. Paul clearly calls Gnostics heretics. I think you are on safe ground then calling them the same.