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!

Yes, my take is that the siblings should add up their salaries - what all three of them earn in total, not including any income from a spouse/partner - and work out who earns what % of the total, and then allocate the estate to the people with the lowest % of the total earnings

so for example, if rich-sister earns 90% of the siblings total combined earnings, rich-sister should get 10% of the total inheritance.