Yup. Yvonne planned to give Penny the glass that wasn't poisoned. The glasses were prepared by Penny. So if Penny was ok after drinking it while Yvonne drank the poisoned one, penny would trigger the "The villainess poisoned the protag and thus she is executed!" cliché aka dead end.
It's messed up. Penny to avoid that death flag, drink the poison willingly. The result is the same: she dies. but drinking the poison is much better than being executed, framed etc. It's very "If I'm gonna die I'll do it on my own term." it's messed up but this is the best course of action to get rid of Yvonne's "I'm a poor girl bullied by my jealous stepsister " trump card.
AGH IM CRYING DHHSHSHS WHYYYYYYY
Im jumping off walls rn man
She gonna drink poison so it doesn't look like she tried to poison her "sister" ?