I'm finding it a bit of a challenge to articulate how flawed these assertions of yours are, despite the vehemence with which you put them forward.

If she genuinely suspected beforehand that they were literal brother and sister, then there wouldn't be a "somehow" involved, would there?
Therefore the meaning I took from it was that she sensed a brotherly connection, without explicitly thinking he was her actual brother, because of course she didn't HAVE a brother, did she? (As far as she knew). Whether she'd had this fleeting feeling of a brother/sisterly love (via the force, female intuition, whatever) she would have soon shrugged it off as ridiculous or impossible. It doesn't mean that the phrase "somehow I've always known" wouldn't have then had validity when she was finally confronted with the improbable truth.

I won't bog down this thread any further. Ady has already said what he's doing with it, and I support it, but not for the same reasons as you.