The only thing Rudolph should learn from this is that he can't trust a single one of his brothers to any degree and that they all would eliminate him after having used him to gain the throne. It's just that sort of family. They don't give a shit about his disinterest in the throne. At the end of the day, he was either trash or didn't exist in their eyes before the game of thrones really started, and Rudolph gained his own backing, so it would be beyond naive to think they suddenly started to value him on a personal level.
I hope Oscar actually is lying about pretty much everything in this chapter. It would be refreshing to have a lying character in manga. I've been disappointed time after time over the years hoping for a plot based on lies in manga, but it seems to never happen. Everybody is always telling the truth, be it good, neutral, or evil characters. Even in situations where a story would have immensely benefitted from a lie, but nah, never. Of course I'm not claiming there are no lies in manga, but when there are, it would be a single story-defining lie the readers would be well aware of. Never in a scene like this one, where Oscar could have simply fabricated much of what he was talking about, yet readers wouldn't immediately know.