I don't think there is any sound argument against incest which does not depend either on eugenics arguments or power imbalance arguments (the latter obviously being invalidated in the siblings-separated-at-birth case).
The main reason I can see for not advocating incest (or, indeed, gay relationships, or women's rights) is the 'be above reproach in your society wherever possible' argument - that Christians (and presumably Jews, although I haven't studied that one as much) should not do anything which would bring them opprobrium from the people around them unless they are obliged to do that thing by a moral imperative, as opposed to just wanting to do that thing for their own comfort. In Christianity this is explicitly framed as being for the purpose of making people more likely to convert. (It's been a while since I looked into this, but ISTR that it's all in the Letters somewhere.)
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The main reason I can see for not advocating incest (or, indeed, gay relationships, or women's rights) is the 'be above reproach in your society wherever possible' argument - that Christians (and presumably Jews, although I haven't studied that one as much) should not do anything which would bring them opprobrium from the people around them unless they are obliged to do that thing by a moral imperative, as opposed to just wanting to do that thing for their own comfort. In Christianity this is explicitly framed as being for the purpose of making people more likely to convert. (It's been a while since I looked into this, but ISTR that it's all in the Letters somewhere.)