There is a 'right' sister. But just as 'Right' and 'Wrong' in real life, its all in your head. The difference between ascending yourself or a sister is the same: Do you want to make one person happy(you) or make the entire world a better place?
Many argue that The Nameless Sister is the 'right' one, but some argue that resurrecting her is 'wrong' because it ruins her act(s) of selfishness.
That said, I think the actual ending is the weakest part of The Void. The choice thing is brilliant, but the poems themselves are a bit lackluster.
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in the original Russian version turgor you could ascend together with a sister.
I'm glad they removed that. It cheapens the choice of ending and therefore the game itself.