Because their Jesus is not my Jesus. They deny Jesus is the Father. They deny Jesus is the Holy Ghost. In so doing, they have made another Jesus than that of the Bible. In order to be a brother, they must have the same Father, and they do not have my Father. They may say I am their brother, but I do not call them mine, because of the scriptures, as found in
2John 1:10-11, "If there come any unto you, and bring not this doctrine, receive him not into your house, neither bid him Godspeed: For he that biddeth him Godspeed is partaker of his evil deeds."
They can be fervant in their beliefs, and still be fervantly wrong. Trinitarians are not my brothers or sisters. They have a different God. One other thing to mention on this.
John 8:24 says this, "I said therefore unto you, that ye shall die in your sins: for if ye believe not that I am
he, ye shall die in your sins." Notice ion the King James that the word 'he' is in italics. It was added for clarity. But, the original says that unless one believes Jesus is the I AM, which means unless a soul believes Jesus Christ is God and God alone, then cannot be saved. Only God can forgive sins, and only God can save the soul.