it was the most stupidest rapist i ever did see.. trying to drug him but ends to be drugged herself.
what next? if she attacks someone.. she hurts herself?
If I remember correctly, from reading the novel some time ago, the reason she drinks the spiked wine is that it has an instant effect, like we see with her.
And when she sees Shiro is fine, she thinks something must've gone wrong and assumes the wine is roofie-free and that she can drink it without passing out.
Also her lower half was probably doing most of the thinking.