To me it make perfect sense why Andras isn't just raping Alexia whenever he feels like. As mentioned she's just some farm girl. There are a million of them out there that he can fuck when ever he wants. She isn't what either twin actually wants, she is just a means to an end and that end is Rowan's servitude.
However Rowan's service is contingent on Alexia remaining unharmed. If your just going to rape her regardless of his actions, there is no reason for him to keep working for you. They need to be able to hold that threat over him to ensure his compliance.
That said, knowing that they can't have her actually makes them want her all the more (and in Andras' case he likely also wants to prove that he is the most alpha, because he is clearly has daddy issues). They can't just take her, because she would just tell Rowan what happened and everything falls apart. But if they were able to seduce her or otherwise make it seem like it was her own choice...well then they could have her and she would be incentivized to keep her infidelity secret from Rowan, the best of both worlds.
Of the two, Andras is by far the most blunt in his attempts. He regularly implies that there will be consequences for Rowan if she refuses him, or lets her know that he could just take her if he wanted so she might as well just give in.
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But when push comes to shove, whenever sh sticks to her guns ans says no regardless of any threats, Andras always backs down. Not because he secretly has a consent kink or anything. But in the end, some random farm girl pussy isn't worth blowing up all their plans for.