Nothing in the Bible identifies anyone except Jesus as a biological child of Mary. Absolutely nothing, not a single verse, nothing.
It mentions that Jesus had "brothers and sisters," but never calls those brothers and sisters children of Mary, and in fact specifically identifies their parents in some cases.
Amazing, and sad, how you can make dogmatic assertions without the tinest trace of Scriptural evidence for it.
Also, Scripture has these same relatives rebuking Our Lord. If they were younger siblings, they would be breaking the tradition of the Jews. The fact that He also left His Mother with the Apostle John "seals the deal."
Some Protestants refuse to accept the fact that in the Mediterranean and Balkan cultures the term "brothers" and "sister" is used for first cousins. In Serbia, for instance, the first counsins are considered "brothers" which is to say "the same blood," so that no possible marriage could take place between them. In fact, the Serbian language doesn't even have a word for a "first cousin."
It's this kind of nonsense that comes out of beliefs based on sola scriptura heresy, imo, taken out of context of the historical, geographic, cultural and lignuistic setting of the Gospels, and misinterpreted in the present-day mindset and New World culture, based on a translation of a translation.