In V13 Jesus takes Thomas to one side to give him some differentiated and individual teaching. He does this because Thomas is the apostle who demonstrates the best understanding regarding what Jesus is.
Additionally in the opening prologue, Thomas is named as the apostle who wrote the gospel down. He is referenced as 'Didymus Judas Thomas'. 'Thomas' means twin in Aramaic, and 'Didymus' means twin in Greek, so our purported author is twice referenced as a twin. Rather than being a biological reference this seems to me to point towards the special bond, and spiritual kinship, that the Apostle Thomas and Jesus shared.
