9 year old doesn't know they are adopted

Hi, I'm sorry this is so long to read, but I would really appreciate it if you could read through, but to really shorten it, my 9 year old sibling doesn't know they are adopted, because my father refuses to tell them or to allow anyone else to. It's really not that simple, but that briefly sums it up! This is my first post on here, I'm not a parent, I am a sibling but I am really in need of advice and would be extremely grateful for any. I will try to be as vague as possible as I don't want to be identified. My family consists of 5 siblings and our parents. I am the 2nd oldest, and of my three younger siblings, the 2nd youngest was adopted at birth (youngest sibling is biological). They are now 9 years old. They do not know they are adopted. My mother wanted to tell them from birth, but my father was completely set against it. Without going into too much detail, what he says goes. End of. He will shut down any normal discussion, close up, because he is the boss. I feel so wrong typing this, like I am blasting my father. But the older I get the more I realise how wrong this is. I love my parents, we have all had the best childhood we could wish for, despite certain issues. Our parents have always gone above and beyond for us. It's just this one issue that has me at a loss. I am completely terrified for my sibling. They don't know they are adopted. The right thing- every professional, people who have adopted or have been adopted agree that it is the best for the child to know. We have had so many arguments about this, and I feel like I am really pushing it and treading on thing ice, because in our 'traditional' family, this is seen as 'disrespectful', but the issue is so serious that I am just terrified for them. Everyone in the extended family knows. One day it's bound to slip out. Even at this point, my father says he will deal with that person, if they tell my sibling. But that's missing the point! I don't care about the person, I care about my sibling and the trauma it will cause. Either way now, I feel like in the best case scenario, a 9 year old will find out they are adopted, in the worst case, they will find out as an adult. Either way I am scared of the trauma it is going to cause them. I wish I could have done more. I was 12 when my sibling was born, and I try to tell myself I was only a child up until the past couple of years. In the past few years as I have grown older, I have been trying to reason to deaf ears. At the end of the day, it is selfish. He is not willing to listen to professionals. He believes knows better. He thinks they are too young to know, they will feel the odd one out etc., I have shown him so many studies, books, what adoptees have said etc. He is just so stubborn he won't listen, and refuses to discuss it. No means no. I hate to even say it, but I feel like when arguments have occurred it is bordering on emotional abuse. Maybe it is emotional abuse when you can shut an argument down by yelling at someone and smashing things up (bear in mind that although us older siblings had witnessed things like this as younger children, the little ones haven't, and I don't know if it's because the occurrence is less now, or because my parents consciously try to argue less in front of them, especially because I have told them numerous times how certain things affected us growing up). This makes our family seem to be a volatile mess! But it isn't, it's hard to explain. He will realise the damage he has caused when it is too late, and I am so scared that it already is. My sibling has mild learning difficulties, which could make it harder for them to understand. They know just how loved and cherished they are, not only by our parents, but by us older siblings as we do spoil the younger two, likely because of the large age gap! My understanding is that adoption should be normalised from birth, so it's not a 'thing'. Please don't blast my parents for this. I feel like I am betraying my family for even typing this, I feel so guilty. But at the end of the day it is for my younger sibling and I would give the absolute world for them. I cant bear the thought that they might go through completely avoidable trauma because of this, and that I should have just told them myself. Then again, I would undoubtedly have been kicked out and never seen them again if I did so. It's not my place to tell them. I'm not the parent, but I love them as much as if I were. Is it too late at 9 years old to tell a child without it being traumatic? I feel that they are starting to realise physical differences, for example how children look like their parents, and whilst the same ethnicity/skin tone/hair colour, anyone can see that physically they do not look similar. Considering they are familiar with adopting pets from the shelter, I don't want them to liken the two, or to think they are different to us or other children, or the odd one out. I'm so confused and worried. If you have any advice or similar experience I would be really grateful. Thank you.