The passage in Mk. 3:20-21, 31 bothered me also for quite some time since, on the surface, it appears that the Blessed Virgin Mary & Jesus’ cousins (or step-brothers, if one is an Orthodox Christian) were against Him. I was finally able to do a more thorough analysis of the gospel accounts and come up with a remedy for this apparent problem. ----- Let’s look at the passage in Mark again:
He came home. Again [the] crowd gathered, making it impossible for them even to eat. / When His relatives heard of this they set out to seize Him, for they said, “He is out of his mind.” // His Mother and His brothers arrived. Standing outside they sent word to Him and called Him. — Mk. 3:20-21, 31
Here, we already see a discrepancy in the aforementioned line of reasoning. The “relatives” referenced that doubt Jesus are already in His presence as of Mk. 3:20-21. “His Mother and His brothers” do not appear on the scene until Mk. 3:31. Therefore, we can see from what is written in Mk. 3:20-21 & Mk. 3:31 that these verses cannot be referencing the same individuals. ----- So, who exactly are the “relatives” referred to in Mk. 3:20-21 that are doubting Jesus and accusing Him of being insane?
I believe that the reconciliation between Mk. 3:20-21 & Mk. 3:31 can be found in Jn. 10:19-20. Here Jesus has just given His teaching on The Good Shepherd. At its conclusion, its reads:
Again there was a division among the Jews because of these words. / Many of them said, “He is possessed and out of his mind; why listen to him?” — Jn. 10:19-20
So, from what is written in Jn. 10:19-20, it would appear to be clarified that the “relatives” referenced in Mk. 3:20-21 were actually Jesus’ fellow “Jews”.