If I'm understanding this correctly, she's the half-sibling (or full) of either the dad or mom. Which means the grandparents had a miracle kid or one of grandfathers managed to get a different woman pregnant?
Tbf I think that's the entire point of the manga. Plute is super OP so she can withstand all of the author's fetishes and the collateral damage doesn't matter bc Plute is OP.
tl;dr: everything is an excuse for the author's fetishes