As for the adelfoi / suggeneis controversy, the problem is that while the latter is a word for 'relatives' - literally meaning one of the same gens or race - adelfoi may also have a much broader meaning than 'brother'. Lot was Abraham's nephew, not his brother. And the 120 'brothers' in Acts 1 are unlikely to all be born of the same mother.
The real problem, however, is that with my Liddell & Scott and 3 years of Classical Greek I can sit here splitting reasons with you forever, but without the Teaching Office of the Church it's not persuasive. I can marshal arguments for my side and you can marshal arguments for your side, but the Truth does not depend on who is the more persuasive advocate or who has a better knowledge of Greek (admittedly I am more at home in Classical than koine). It's like two lawyers arguing -- there has to be a judge. And the traditions and teaching of the Church, for me, are the arbiter that fetches the most ingenious argument from Scripture up short like a canny old Superior Court judge, "Yes, that's all well and good, but THIS is what the law says."
Or, as an old Scots Session judge said to a plausible criminal, "Ay, that's all verra weel -- but ye'd be none the waur of a hangin'!"
Brother, in Greek as in English, can refer to a fellow believer or very distant relation. Context determines. So if I say, “My brothers were attending a bible study”, I could be talking about brothers, or fellow believers. If I’m filling out security paperwork, then I only list blood brothers.
So when someone says ‘his mother and brothers are here’, it strongly suggests brothers by family ties, since Mary is linked to them. Likewise, James, brother of Jesus - when mentioned in Galatians 2 - obviously means something more that ‘fellow believer’.
The real problem, as you suggest, is you need to follow your church’s teaching. If scripture plainly suggests otherwise, you still need to follow your church.
On a matter like Mary’s perpetual virginity, I don’t care all that much. You obviously can be a devout believer and also believe in her perpetual virginity, or not.
On other matters - Purgatory and Indulgences - it is a bit more of a challenge, but that is a subject for a different thread.