No, Scripture doesn't say his siblings are Mary's children ANYWHERE. In fact, Acts 1:14 doesn't group Mary as the mother of Christ's siblings, but lists them separately after saying she is the Mother of Christ.
Your reading of Scripture did not exist five hundred years ago, a thousand years ago, fifteen hundred years ago, or two thousand years ago. It is a brand new reading, you "developed" your understanding and changed it, just like the Papists. And you will continue to do so, year after year. A hundred years from now, Protestantism will be still more different in how the Bible is understood, in ways that are at odds with today's Protestantism (and the "new understanding" will be called "conservative" when in fact it is innovative, just like the new understanding of today is). Your philosophy is one of change, like the Papists, one of doctrine improving and becoming more accurate; the only difference is you use "improved" understanding of the Bible to justify it. If the Protestant Church were guided by the Spirit of Truth, you wouldn't waffle on your doctrines, you would have consistent, unchanging doctrines. But your doctrines are not consistent and unchanging, you keep changing them! What's more, you teach contradictory dogma in different Protestant churches! How can you claim even remotely that the Spirit of Truth is guiding your Church when different, contradictory dogmatic truths are being taught within it? And they change over time?
There is One Spirit, One Truth. And He doesn't waffle, and He isn't a cacophony.