It was Great Uncle's right to leave his money to whom he wished. But elder family relatives who set up this unhealthy scapegoat -v- golden child dynamic between siblings must know they are very likely going to damage the relationship between those siblings, probably permanently. And they either don't care, or it's a state of affairs they consciously bring about.
Okay so the sister is a mercenary woman who reneged on a promise no one asked her to make, but it's the Great Uncle's behaviour which was, IMO, wicked. People like him would be happy from beyond the grave if they knew how effectively their money has brought about divides in the families they've caused trouble for throughout their lives. And yes, they do know exactly what they're doing. I've seen this sort of thing before and with similar results.
And of course, that puts OP in a hell of a position, doesn't it? If this did cause divides in the family the sister can project the blame directly onto OP and say this was her choice, and just by the way, isn't she the mercenary, greedy one to whom money apparently matters more than family? You can predict her sort of protestations from a mile away, but the trouble is when it comes down to money they are pretty difficult to counter.
I just don't know how you can win this one, OP. I'm sorry you were burdened with such a pitiless, unprincipled sister and Great Uncle. But I would comfort myself with the knowledge that I wouldn't want to touch his money.