Again, I'm not saying the characters are one and the same, just that they both exist precariously around the edges of an established fiction. Yes, the sisters were already a thing previously, but as a part of back-story, not as fully-fledged characters (barring Aigis). To use that as the basis to automatically give a tenuous link to a new character shouldn't give them precedence.
Though this varies to an extent, I still think that it is wholly reasonable to resent them for the way they force themselves over the fiction. I professed some endearment towards Marie, but I can't deny she is contrived in her placement and prominence, and I feel the same applies for Labrys too. Even is she exists as part of some side-note in the pocket of spin-off media previously, her transition during Arena still feels jarring and uncomfortable. But that also exists as a larger criticism of that games story.