...because they ask for a moral intuition about a case where the outcome is predefined.

One thing i found a bit dodgy about that example is that it just asserts that the outcomes were positive.

I would bet that, for the respondents, simply being told that the outcomes were positive would still have left them feeling that in a real brother-sister situation like that there would likely have likely been some negative consequences.

Greene does not seem to factor this into account when he interprets their responses.