I don't know that the MC will necessarily be able to change anything with Sam or Aubrey, but I think they're both bi. Sam previously had a boyfriend, currently she's with Aubrey, and she's attracted to the MC sexually. I think Aubrey, by being a succubus, is by default bi, since the demons drain life from humans generally, not just limiting it to a specific gender. I would assume she's been in relationships with men as well and not just with women, but obviously I could be wrong.
A potential way I see of the MC moving forward with both is that he could dominate Aubrey when he's strong enough as a wizard. There have already been multiple comments about his aptitude, and he's a pureblood. So I think he might eventually become too powerful for Aubrey to manipulate. If he then manages to romance Sam away from her, Aubrey will lose her source of nourishment, which the MC could become. But there would be stipulations, limiting how much she can get from him, protecting his own ligaments from damage. The stipulation could easily move into domination in the sheets as well. (Or, at a minimum, he might be able to remove her dominating attitude during sex.)
However, for the MC to get to that point, a lot still needs to happen. I guess we'll see.