I find the sexual tension weird.. like aren't they siblings or am I mistaken? and tbh I do...

they are half siblings, but as fontaine said/thought before, its not odd for this family to marry between half-siblings

Sexual tension? Since when?........ Has it always been there?

Well yeah, it’s been implied that she slept with dion and the other step bro Fontaine. And apparently dion always had a thing for Roxanne, so once u knew, u felt the tension...

correction, dion says he wants to see her cry but more like wants her to dote on him, so she's guessing he wants to have sex with her too, but they haven't. he's acting as her loyal dog as long as she gives him the bare minimum of affection (even when she says she hates him, she still pets him in weird ways).
fontaine totally wants to fuck her, and is willing to marry her when he becomes the heir (just his delusion though). she is giving fontaine mixed signals to have him act as she wants him to.
the only one she's slept with so far is the ML that one time, she just makes others want her so that they'll be willing to do everything she asks for, for a chance to be with her

First thank you for that. Second I thought she didn't sleep with him only left a few marks on him.

I knoww! it was really unclear, I read that part at least 3 times to understand haha