Jesus told His brothers that His time differed from their time. They could come and go without any significance — for them any time was right. He, on the other hand, always pleased His Father, God, so His time movements were those that His Father desired.
How many signs would have been sufficient for them to believe? Jesus had already performed a number of miracles there in Capernaum (i.e. a man in the synagogue with an unclean spirit (Mark 1), Peter’s mother-in-law (Mark 1), the sick and demon possessed (Mark 1), Centurion’s servant (Matt. 8)) What is necessary for us to believe?
The apostle John wrote in 1 John 4:18, “There is no fear in love; but perfect love casts out fear, because fear involves punishment, and the one who fears is not perfected in love.”