PLS HELP ME FIND THIS BOOM

Look its a book about two one brother and one sister who tries to kill each other and uuh I remember seeing it in like elementary which was interesting ... but the cover was like the two siblings one boy one girl, it was in dark colours and ine of the siblingss was on a stair, one of those bookshelf libary stairs those who can like move yk? And the other sibling was like sawing the leg of the stair, while the sibling that was ON the stair.. uhh lemme see how to explain, the was a weight OVER the one sawing and the person on the stair was abt to cut it. the vibe was kinda gothic i just remembered it bc it reminds me of the coffin of andy and leyley THATS OFF TOPIC, if I remember right the sister looked like Mavis Dracula istg, I'm from Sweden maybe it's a Swedish book idk, is this too niche? Anyone please help me!!🫩