I appreciate your openness to Catholicism.
I am a convert from atheism, over a very long period of rational investigation of many different religions and philosophical standpoints. I found that Catholicism alone can perfectly answer all questions and fit the world most seamlessly. As such, I, and I'm sure many others in this group, are more than willing to discuss anything you wish about our Faith.
As for this specific question, I think it is important to understand that the idea of sola scriptura, the belief that all Christian doctrine and discipline must derive from the Bible, is purely an invention of the Protestant Reformation. Indeed, the New Testament was written and organized by the Church, which had been alive for decades before any book of the NT was written and for centuries before it was definitively compiled. This is also true of the Old Testament, whereby Judaism had existed for a very long time before it was written or finished. The Jews, like Catholics, understood in the time of Jesus and before that the Temple and the Church, not the Bible, are the living centers of the Faith, through which we actively live God in this world. The Bible itself, especially the NT, testify that oral tradition and the authority of the Church are equal to and often precede the Bible. (By the way, if you would like any specific quotes about these statements, just ask; I dislike citations and so I rarely give them unless asked.)
The doctrine of the perpetual virginity of the Blessed Virgin derives from several things. For one, it, like the Assumption, had been apart of Christian tradition since the very early Church and I believe many of the Church Fathers attested to it. For two, it is based on the Catholic sacramental imagination, whereby things in the Faith fit not just fact, but the symbolic and spiritual truths of God. For example, only men may be priests not only because Christ only chose men as apostles, but because in their liturgical functions, priests act in persona Christi, meaning in the person of Christ. They act as Christ and act in a "giving", rather than feminine "receiving" sense; this is also why the Father is called Father, when in truth He has no gender.
These are some of the main reasons for our belief in the perpetual virginity of the Blessed Virgin. As for the siblings of Christ, Koine Greek had no name for cousins or other similar relatives and so called them brothers/sisters interchangeably. The Church has always taught its use of the term siblings refers to Christ's relatives, not his brothers or sisters. Again for specific verification on this if you wish, just ask.
Thank you again for your interest. Though I am certainly not trying to proselytize you, I would advise you to reconsider your position that you will "never be a Catholic". If you are truly open-minded, then allow your mind, your heart, your conscience and your entire being guide you wherever Truth wills. This was my method and it led me home. God bless.