Proof that she shares a slot with Marth? Aside from the fact she wields a sword and has a similar build, she does not take after Marth in much respects. Marth fights with a fighting style similar to a one handed rapier, with slower, more forceful strikes, while Lucina appears to use more of swift blade-storm style.
I'll hand it to you they do look similar, and it may prove they share the same slot, but Lucina appears a lot lighter than Marth, as well as more aerial inclined. Of course, I've only seen 10-12 seconds of her fighting in Smash, but she is different enough from Marth to constitute her own slot.
In addition, Sakurai seems to be leaning towards moving characters away from accompanying the same slot.