You have to accept the conceit that Hozuki is a genius beyond genius. On the level of one of the greatest minds humanity has ever known. That does help in understanding why Kazuki resents her. Aside from her combat skills that nobody notices, she's totally normal, as is the eldest sister.

Hozuki got around to purchasing weapon systems, ammo, armor plating, etc., and had them delivered at home without anyone noticing. She cracked the passcode on the mysterious warehouse connected to their home that they've had for decades, but no one opened.

Is suspension of disbelief high? Yes.

I agree that if you can't get past that, it's going to be hard to like.