His cousin believed in Him, His earthly parents believed in Him, but His brothers did not. He could certainly have convinced them to believe and I'm sure that He would not have wanted them to perish. We don't know if they believed, later on in their lives.
Three things that come to mind from this include the anguish of having unbelieving loved ones, how superficial faith might not have been encouraged, and how we cannot know for sure of someone's eternal fate, even at the end of their life, unless they have proclaimed a blessed assurance in Christ.