Bedrockgames
I post in the voice of Christopher Walken
See, this is where I get confused, because this is a contradiction.
How does the GM unilaterally making this decision---the brother of Player X's character is already dead---not, by its very instantiation, thwart any and all possible choices Player X might make that relate to the player's desire to explore the relationship between the character and the character's brother?
Because in a game like this, getting that specific about the types of outcomes that unfold in play for things that have yet to happen (i.e. finding your brother and having a relationship with him) is not considered a reasonable choice. The choices that matter for agency in this style (and this is pretty universal among sandbox gamers) is the choices you make within the setting. Choosing to look for your lost brother is such a choice. And if the GM suddenly thwarts all your efforts to find him, then that would be a kind of railroad or a violation of agency. However it is perfectly reasonable for the GM, who is in control of the setting, to decide that your brother, who is part of the setting, is dead. Once you find your brother, then you can choose how to respond to the situation find. It would be really strange in this kind of game for the player to be able to set goals that get into controlling the setting, and then get mad because some setting element didn't end up being what he or she wanted. Again, you don't have to like this approach. It might not be for you. But i don't think it is all that hard to appreciate what is going on, what kinds of agency is on the table, and how within that style of play, you are making important choices in the setting.