Their designs are elegant and unique! Don't make such shallow comparisons just to ridicule and underrate their appearance! And they are better as twin sisters instead of one character. Just 1 wouldn't be nearly as interesting.
Well, okay. Barring story, they're gimmick characters who can fly.
And when including story, their mode adds hardly anything to it. Their perspectives are completely generic. The events shown are already known about in the main story.
how can Castlevania fans think like that..... -.-; Anyone can make ANYTHING sound dumb, plain and nothing special. Just as someone could say Sypha is a gimmicky magician that wields a stick, or Simon is a generic warrior that wields a whip, or Alucard is a gimmicky vampire guy that can transform and do other generic vampirey things, or other ignorant things like Trevor is a Simon ripoff.
There's no character development at all to them..
First of all, not all great characters need character development. After a certain age, people are who they are
and not everyone goes through life changing experiences that greatly change the way they think about things in such a small timeframe.
That doesn't make them less of an interesting person. It's interesting you say there is no character development at all to them, when Alucard even at the finale of SOTN thought and felt the same way he always had, Nathan was the same person, Charlotte and Jonathan were pretty much themselves the whole way through, Simon doesn't even have character interaction or hint of a personality, and they, like Trevor, are still extremely likable characters, etc.
Even the floating gameplay is at dissonance with how they're human
It is unknown what type of magic they learned, but as Maria and Shanoa have proven, there are more types of magic that exist in the Castlevania universe than what is known. It's just as believable that human Stella and Loretta can float just as it's believable for unique humans like Belmonts, Belnades and Morris families to possess abilities beyond that of normal humans. Their generation of Lecarde would be no exception. Also they can't float indefinitely like vampires can, so that ability is not out of place in their story's gameplay.
It's really nothing to write home about that they just decided to include two or so menial cutscenes to an extra mode.
Menial? MENIAL??!?!

Just because you have no interest in them doesn't mean their value should be overlooked.
If there is, you should be doing it about Dawn's Julius mode then. I mean, Portrait of Ruin's Sisters Mode has an ending with dialogue as a point over Dawn's Julius mode if anything. But again, it just recaps things you already know about, and adds nothing of value to what we already know.
It does add value! It's worlds of a difference between experiencing their journey to rescue Eric, than it is to just hear about Stella and Loretta's past through other characters.
It not only adds more story, but also adds more meaningful gameplay as well!