Jude identifies himself as the servant of Jesus Christ and the brother of James.  James was the oldest of the half brothers of Jesus and the head of the Jerusalem church.  In Matt 13:55 and Mark 6:3, the half brothers of Jesus are listed by name and Jude (Judas) is included.  It goes on to mention sisters as well.  This should also dispel the notion that Jesus was the only child born to Mary since the scripture specifies several brothers and sisters.   However, Jude doesn’t mention his natural relation to Jesus but rather his spiritual relationship to Him.  He does identify himself with James, his brother, because James is well known among the churches, and this gives him credibility. 

This book was not accepted into the Canon of Scripture right away.  Due to references to other writings that have not been accepted as Canon, the epistle of Jude was held in scrutiny until about AD 350.  Jude refers to the book of Enoch and the Assumption of Moses in making his points in this epistle.  He is not referring to these works to support their inspiration but rather to support his point of discussion.