But I could see her being the girl's older sister. Then it would be Sam and Dean against Bela and Lilith, another way to point out that Bela is Dean's counterpart.

Oh true, good call. I can see that.

If she really was a part of the main plot, it would at least somehow explain her sudden presence (besides Ostroff's obsessivness with anything with a vagina that is).

Also true. I just am beyond the stage of frothing rage and into blinding apathy with her. I just want to fast-forward her scenes and be done with it. :)