Yep, I knew this character would just exist for the sake of being a massive filler/distraction as soon as she called them "toys" after figuring out their in-game identities. Authors running low on material is always sad, but the first 11 chapters were glorious...
Seriously, the worst character in sibling incest romance is always the third wheel who's attracted to one of them, notices their relationship, and says "oh, they're in love with each other, but they're siblings so they can't possibly find happiness, thus I'll insert myself between them and expect that person to choose me eventually once they give up on their sibling". It usually just results in less interactions between the siblings, and lots of insecurity and self-doubt, stretched out indefinitely for as long as the author feels like it.