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I think your first approach (attempt to recreate situations between the brother and sister where emotional connections were formed) would work better.

In such cases, I wonder whether appealing to someone's logical side would be effective since many of these cases are rather illogical. For example, you could get a DNA test to prove that the woman is his sister, but would he be convinced? I speculate that he would find some illogical reason not to believe the hard evidence. It's frustrating because it would be easier to appeal to the logical side if that strategy worked...