I've sympathy for both sides, TBH.
The fact is that a gift of [say] £10k would make a hell of a lot bigger difference to the life of a poor person than to that of a rich person.
"Equally" and "fairly" are not the same thing.
To take an extreme example, if a couple had two kids, one of them a lottery jackpot winner, one of them not, it'd frankly seem a little odd, wouldn't it, to leave them each bang equal amounts in your will, especially if the value of the will was small relative to the value of the lottery jackpot.
And don't many forms of uneven wealth between siblings involve at least some element of 'luck' - unequal intelligence levels, unequal choice of spouse, [etc]?
If we're talking about a poorer sibling who was born with every advantage [e.g. brighter, got sent to a better school, got given small gifts of money, etc] spent her life [e.g.] having a whale of a time constantly on holiday or whatever then that's one thing, but how often is it like that?