There are a LOT of products that bridge over that sexist line just fine. Cars, phones, toothbrushes, they're all geared towards certain demographics, ages, and genders. One could make the argument "Well why not bridge it with boy and girl styled blasters?" This is likely where the Rebelle idea got started. To me, it just doesn't need to be for all the reasons I've already pointed out in this rant. Girls that typically want to shoot at things are already fine with it. Making a product "for girls" that 'enables' them to play Nerf seems sexist already, as if the blasters and the game itself aren't fit for a girl to play unless it's pink and looks like a pop-star tatoo.
I might be wrong and the Rebelle lineup could explode into the Barbie aisle and be a big hit, but I don't think it really needs to. The girls that are already playing will likely pick them up, but I don't expect this lineup to branch out to other girls who hadn't considered this before, which is the whole point of a lineup like this: to reach out to an audience you haven't tapped into yet. Nerf already HAS tapped into this with their "boy" centered products thanks to badass girls that are cooler than their stereotypes.