When they were kids they got along rather well, for the most part! It was after their parents died that they started to spiral. Having to witness their parents dying and having to take care of themselves really fucked them up. Ynx started taking his feelings and frustrations out on others which lead into a rabbit hole of violence and perversion; he started researching and admiring human serial killers and began mirroring them. Yeep more or less goes along with Ynx's insanity because she's desperate to have a family and keep them together. Making the pupa with their father's old tech was her way of trying to solidify that they have a family again. She's so desperate for a family that she more or less turned to incest and trying to please Ynx and participate in his life. It's very toxic, but mmm, it hits all the right flavors.