If I could prove that I wasn't crazy or sensitive and that there was something tangibly ""wrong""
This is worth picking apart with your T for sure. Because even though your motivation is to somehow change him, you're proposing that you go about it by changing yourself, and the way he perceives you.
My two cents (take what's helpful, leave the rest), abuse doesn't stop because the victim becomes better, or more persuasive, or any other thing. Abuse doesn't stop because the abuser sees "well shit, I guess I really did hurt you". You couldn't have stopped it as a child by being "better", there's no reason to think that you could now.
If you think that your siblings are being abused, there's ways to address that (raise it with your T, and do it with their support behind you). Granted, in many parts of the world, the systems in place to protect at-risk children are shit. But the systems do usually give you some degree of protection as the person intervening.
Your job now, your number one priority? Is healing, and getting to a point where you're living your best life. Not being sick enough, or damaged enough, to somehow persuade your abuser to change his ways. Get well, and do what you need to do for your siblings in a way that protects your own healing. You will be best placed to help them if you help yourself first.