BEASTLY is the title of the novel mainly focused on the two children. It is FAR MORE DARKER than SNAKE HOLE. THIS IS WHAT YOU DESCRIBE IF OBSESSION AND GUILT WILL BE A PERSON.
Beastly follows Cyrus and Cecil, the twin sons and the other sibling is not part of the main characters. As their lifelong bond grows into something far more intense and complicated than ordinary sibling affection. Being part snake and part human, they struggle between human morals and their own instincts, with Cyrus openly devoted and possessive of Cecil while Cecil battles guilt, confusion, and fear over his feelings. Most of the story focuses on their emotional turmoil, dependence on each other, and attempts to understand where they belong as beings who don't quite fit either the human or beast world. In the end, after much internal conflict, they choose each other and accept their nature, resulting in a happy ending where they remain together despite the challenges and taboos surrounding their relationship.
here if you wanna read:
https://www.novelupdates.com/series/beastly/

if you think this is freaky enough, wait for the children's story THE MOST FREAKIEST AS FUCK