As I said elsewhere, I think identifying blackmail depends on identifying an agreed status quo state. This is fairly easy in ordinary life (if we had an affair, the ordinary human expectation is that you would keep the letters secret for free, etc.), but more difficult in outré cases like this one and the other one you suggest with the letters in the cave. It is not surprising therefore that in cases like this we do not have clear intuitions.
That said, following the definition I proposed, I’d say that in your first example if it is a plausible status quo that your brother would go to Olympos (maybe he got a job offer letter form there?) then it is blackmail for you to go instead. If he just happened to come up with the idea of leaving, then my intuition is that this is not a plausible status quo; there is no stability, no ordinary human expectation that you would abide by his decision of leaving you stranded. A more plausible status quo (which applies also to the perfectly symmetric case when both come up with the idea together) would be flipping a coin to decide who leaves. Relative to this, either of you leaving is blackmailing the other.
As I said elsewhere, I think identifying blackmail depends on identifying an agreed status quo state. This is fairly easy in ordinary life (if we had an affair, the ordinary human expectation is that you would keep the letters secret for free, etc.), but more difficult in outré cases like this one and the other one you suggest with the letters in the cave. It is not surprising therefore that in cases like this we do not have clear intuitions.
That said, following the definition I proposed, I’d say that in your first example if it is a plausible status quo that your brother would go to Olympos (maybe he got a job offer letter form there?) then it is blackmail for you to go instead. If he just happened to come up with the idea of leaving, then my intuition is that this is not a plausible status quo; there is no stability, no ordinary human expectation that you would abide by his decision of leaving you stranded. A more plausible status quo (which applies also to the perfectly symmetric case when both come up with the idea together) would be flipping a coin to decide who leaves. Relative to this, either of you leaving is blackmailing the other.