For one thing, Jesus’ brothers and sisters rejected him as the Savior, the Messiah of Israel. John didn’t.
And in answer to the thought that one of Jesus’ brothers or sisters must have written something ... I would say that most likely not the sisters of Jesus. They didn’t have women’s lib in those days ... :-) ...
In regards to the brothers of Jesus, they very well might have written something, but since it wasn’t SCRIPTURE that they wrote, it probably wasn’t preserved, in the way the inerrant and infallible Scriptures were. It might have been written and lost in history like most of the other human writings of the day would have been!
It is widely considered that the Epistles of James and Jude were written by brothers of Jesus. Well, half-brothers.