Girl blind asf or is she ignoring her feelings
I think it's because Yvonne is 100% sure that Elsa would end up with one of the male love interests as was shown to her, she still thinks that she's the villainess and hasn't even thought of the possibility of the roles changing or not even there in the first place and that she can be one of the love interests (the main one at that). And rather than ignoring her feelings, I think she doesn't even know they are there considering the fact that she's stuck on the logic that the heroine will end up with one of the male interest and that the villainess and heroine aren't compatible, can't be together.
Yvonne be like: Hmm yes this girl who has sticked close to me, lived with me, taken care of me, has outright confessed her undying love to me, and has blatantly confessed to wanting to do "it" in the future must be straight for the male love interests! 1!1!1!