The point im getting at is Go to the source (The Bible) To see what it says on the issue. And yes Jesus had siblings that's not a mystery Luke 8: 19 "Now Jesus' mother and brothers came to see him" Mark 6: 3 "Isn't this Mary's son and the brother of James, Joseph, Judas and Simon? Aren't his sisters here with us?"

Now as for the His childhood not being written about completely there is one about Him when he was twelve teaching in the Temple Luke 2: 41-52. One can also conclude that Jesus stayed in one area throughout His childhood due to the reaction He received from His fellow townsmen when He claimed he was the Messiah. (He was rejected) Luke 4: 14-30