I got some excellent responses on my question. I don't like to leave question marks floating out there in space, it may land in someone's dream. So here's some ideas to take with you.
Toni reckons that it may have something to do with the detox effect of cilantro, so what someone calls an allergy is actually toxins releasing from the system causing them to feel off.
Vanessa thinks there could be the smell component that turns people off since the herb has quite the aromatic presence.
Which adds fuel to my already established desire to revolutionise and re-define words. The word intolerance when used in context to foods is similar to the word syndrome in response to a medical condition.
We are used to seeing therapists that tell us that that our current crazy is a past trauma, so very specific to our personal history. And yet, when it comes to healthcare/wellbeing, we are using more and more vague terms.
When you break things down into qualities you start to see how each person can derive an issue from an different stance. As Toni said, you don't really know where the person is coming from without asking the details and the context - you said it sister.
Here's to asking what people really mean.