The Assumption of Mary as the God Bearer?

If it was an eye witness account, i suppose some one would have been informed about that. But you don't find the first denial of mary's assumption until in the late, early middle ages. That seems to be something that went without contention. Since since the catholic church is certain that the brothers of jesus are anything other than biological siblings. A, meann uteron siblingsa. Why aren't they just as certain specifically who they are?