My lazy life as the protagonist's brother

I died while protecting my sister from a car crash.
However, instead of the peace and quiet I was longing for, I reincarnated in another world. The Eyes of Rose, to be exact. It was originally a BL novel about the main protagonist, Rian, who had eyes as red as roses. It was a symbol of being cursed with black magic, and his family hated him for it. He was isolated and the servants constantly mistreated him. It was only when he arrived at the magic academy that things took a better turn, but those eyes once again caused trouble for him when they attracted the people around him, especially the prince, who continually abused him because he was uncertain of his feelings. Finally, the protagonist went crazy and died. The end. It was a rather sad ending, and my sister ended up bashing the book to pieces. Unfortunately, I had reincarnated into one of the book's characters. Not as the protagonist or the male lead, but as a side character. An irrelevant side character that was barely mentioned throughout the book. The elder brother of Rian, Augustus, who ignored Rian's existence and lived happily ever after in another unknown kingdom. Except for the part where he said goodbye when he left, there was no interaction with any of the characters.
That's fine. no. That's great! I can enjoy my rich, relaxing life until my/Augustus's betrothed came to take me away to another kingdom. At least...that was what I thought until I laid my eyes on my little brother. Yeah, you can guess what my reaction was. Me, who was a sis-con, now became a bro-con. Don't worry, Rian! This big brother will save you!
But...the plot somehow went off track?