Okay, lemme explain:
The eldest brother, Fontaine, likes Roxana sexually (not romantically, which means he only sees her as an object of his lust) and wants to take her in as his wife since it was normal for their family to marry their cousins and half-siblings in the past. That's why Roxana was able to sweet-talk her way to manipulate Fontaine into doing exactly what she wants him to do. In short, she could manipulate him easily because he was attracted to her and also because he wanted to defeat Dion in something, so Roxana also made sure to take advantage of his weakness (which is his inferiority complex towards Dion).
On the other hand, Dion doesn't see Roxana the way Fontaine sees her, that's why Roxana said that it would have been better if Dion was also sexually attracted to Roxana since it would also make her work easy and manipulate him the way she manipulated Fontaine. But since Dion doesn't feel that way, that's why he gets angry when Roxana hoped for him to lust for her, just so she could use him. Of course, Roxana hates Dion to the very core and would not allow anything intimate to happen between them.
As for the alliance, Roxana and Dion are teaming up (along with a few other Agriche siblings) to take down their evil father and bring the downfall of the Agriches. In return, Dion wishes to receive her attention and die from her hands since only Roxana can make him feel emotions.
sexual tension is not just for lust in bed. Sexual Tension can be anyting.
*Enjoying torturing someone* is also sexual tension.
*Brutal Killing* is also sexual tension.
whatever you feel relieved and also pleasure is sexual tension. whatever way is that.
as for Dion case. Roxanne isn't easy to cry infront of other. But Dion having a pleasure when he seeing Roxanne in despair. When he see Roxanne break down, for him is like seeing a pleasurable scene. That's his abnormality
ps: i really want to find what kind of disorder that but i cant find the exact word
I'm gonna close my eyes and pretend she didn't sleep with her brother ( ̄へ ̄)