There are two kinds of fairness at play here: the "fairness" that says all three of you should get an equal share of the estate, and the "fairness" that says the three of you should divide up the estate based on need rather than maths.
You'll note that neither of those ends with your brother getting less!
LW you can't do anything about the fact that the three of you each have different money needs, but I do understand that it's tough to hear your sister talk about giving up money when that means something very different to each of you. And I know that an abstract sense of fairness is tough when it's your rent money or mortgage on the line. But you can't do anything about that; you can do something about your brother getting less money than you, and I hope you do.
Honestly though: before you do anything you and your sister should probably both talk to Jack together about the estate? It sounds like this might end up with you resenting Jack because of something your sister guilted you about, which I don't think either you or Jack would want. It could be Jack is staying quiet because he values his good relationships with you more than he cares about the money, or (given the age difference, and it sounds like you're still young enough that fifteen years is important) he might even honestly agree that you need the money more than he does, or he's still angry enough at the parents that he doesn't even want their money. It sounds like the three of you have a generally good relationship that sharing your real feelings about his being disinherited won't break anything more than the not talking about it is doing. Sit down and talk it out.
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You'll note that neither of those ends with your brother getting less!
LW you can't do anything about the fact that the three of you each have different money needs, but I do understand that it's tough to hear your sister talk about giving up money when that means something very different to each of you. And I know that an abstract sense of fairness is tough when it's your rent money or mortgage on the line. But you can't do anything about that; you can do something about your brother getting less money than you, and I hope you do.
Honestly though: before you do anything you and your sister should probably both talk to Jack together about the estate? It sounds like this might end up with you resenting Jack because of something your sister guilted you about, which I don't think either you or Jack would want. It could be Jack is staying quiet because he values his good relationships with you more than he cares about the money, or (given the age difference, and it sounds like you're still young enough that fifteen years is important) he might even honestly agree that you need the money more than he does, or he's still angry enough at the parents that he doesn't even want their money. It sounds like the three of you have a generally good relationship that sharing your real feelings about his being disinherited won't break anything more than the not talking about it is doing. Sit down and talk it out.