First, I think you were fine, considering the position she put you (and Sara) in. But I do think there were better ways to handle it, especially since it backfired and ended up putting a spotlight on something Sara doesn't want spotlighted (again, not your fault).
One after-the-fact question to ask yourself is why you felt you needed to intervene in the first place. If Sara is low profile on her abstinence, chances are this wasn't the first time she'd encountered this behavior, and it won't be the last. People can be such boors about this issue. So is it possible you pre-empted her own handling of it? Have you asked her if your response was helpful?
Humor can be used, but that's really difficult when you're gobsmacked! I had a somewhat similar situation once at a dinner party, and I told the offender that *I* wasn't going to drink any more wine until *she* finished off the brussels sprouts (she was allergic to some of the ingredients I put in the casserole).
Also, on OG, I think you're well within your rights to be "harsh" with her now that the event is ended. Her statement that she wouldn't stop yapping about it until Sara drank was stupifyingly rude, insensitive and cruel.
Personally, my convo with her would be to point this out and then tell her that, because of her actions, she would not be invited to future events at my home. She took it a lot further than simply "trying again" to offer wine, which is bad enough. I shudder to think how she'd behave around someone with a severe peanut allergy.