I personally can't document #2, sola scriptura. Others around here seem to be comfortable arguing the point. But I do accept it.
If I don't, I have to determine who is right regarding many extra-scriptural pronouncements. That would give me a colossal headache. (Case in point, do you accept every point that every Pope has claimed as from God? I am assuming you are Catholic.) So I do rely on Scripture alone as my guide. (I will admit, there are so many interpretations of scripture floating around that that in itself is a slippery slope.)
As to point one, while there may be no evidence that Jesus' brothers (and sisters?) were born of Mary, you cannot show me scriptural evidence they were not. The fact that Jesus' family is referenced in Scripture seems to me to be inclusive.
I am not a Catholic, so I do not necessarily accept the Catholic interpretation of God's Word nor its doctrines. But I would never engage in Catholic bashing.
At least those very few declared as infallible teachings on faith and morals because I believe the church is the pillar and ground of the truth.
As to point one, while there may be no evidence that Jesus' brothers (and sisters?) were born of Mary, you cannot show me scriptural evidence they were not.
So as far as point one, to declare Mary gave birth to additional children cannot be determined conclusively one way or the other from Scripture alone. To claim that she did is extra-scriptural.
I personally can't document #2, sola scriptura. Others around here seem to be comfortable arguing the point. But I do accept it.
If I don't, I have to determine who is right regarding many extra-scriptural pronouncements.
Regarding point 2, Scripture does not declare Sola Scriptura as doctrine, and since you believe in it but can't document it, you've determined that someone was right regarding this extra-scriptural pronouncement.
Non-Catholics accept a number of non-Scriptural teachings as doctrine and it always amuses me when they criticize Catholics for doing the same (note that I am not saying you personally are criticizing Catholics).