Sorry I don't get it. Assuming she wasn't the OW, what's the issue? Your ex has a wife ... big deal.... you haven't mentioned any actions on her part that suggests she is any kind of actual problem...
you're not analysing every other guest there?
I'm not sure you can trust what your DD says about wanting/not wanting her there, as you clearly have such negative feelings and discomfort about her, she may well consciously or unconsciously be trying to reassure you.... and the majority of your post is not about the aunt's disappointment or your daughter's feelings, it's about your sense of discomfort.
Try to focus on your daughter - it's her event. I suspect her worrying about your feelings about her SM will spoil her day way more than having her SM there. (Sounds like she has been putting off telling you?)
Sorry if this sounds harsh but I say this from the point of view of being a divorced mum, whose Ex left me for a twenty three year old - 17 years his junior- who is now the mother of his second child. My daughter has no clue about my feelings or what happened. Every birthday I have them over to the house. I celebrate every cute picture and video of my DD's new sister (we don't use the phrase half sister). And all of them are included in the end if year photo book I make for my daughter every year,
Is it tough? Yes. But my daughter's sense of harmony is what matters. Her ability to welcome new people into her life without guilt or worrying about my feelings...
I hope you can just go with the flow and focus on the joy of your daughter's graduation.... that is the best way your Dd will enjoy it too! All you are being asked to do is be in the same space as someone who hasn't, as far as I can tell, done anything awful to you or your daughter?
Much better to have a SM who cares and bothers, I'd say.