This looks like such extremely bad writing. As forced as possible just to be as dramatic and get as many cheap tears as possible.
Everything is pointing to the biological older brother being like that because people were trying to get to the money left behind to the two siblings. That's the most obvious explanation we've been given. However, that makes absolutely no sense. That's like if someone once threw away the food I was eating by mistake, so therefore, I decide to wipe out all of humanity to never let it happen again. It's cheap, nonsensical, over exaggeration. What's more, apparently, he instantly changed the very day that he found out that their parents passed away, immediately after being told.
The only way for this story to make any kind of sense is if the biological brother had been somehow coerced into being this mean to the sister. Even then, there needs to be a good reason, and it needs to not look like it came out of absolute nowhere. If it comes out of nowhere, that's just bad writing in itself.
I just feel like this is actually bad writing that the author cannot redeem.