My sister fell for something like this. She's Oxbridge educated, but not very streetwise, and was a bored SAHM, looking for something that was "her" rather than "family".
When she asked me whether I thought it was a good idea (about 3K start-up costs) I said she didn't strike me as the sort of person who would want to be continually pushing sales - she's a pretty sensitive person, and not pushy usually. Certainly not one to exploit others, or talk someone into spending money they didn't have. She didn't take that on board though. I used to cringe when I saw her totally out-of-character posts on FB, and the way she would try to turn conversations around to market her products. People would be kind and polite, but I could see that she was getting annoying to some.
Her husband also took a gentle approach, and encouraged her to set up a spreadsheet to keep track of costs/expenses. It lasted a couple of years before she could see the bottom line was never going to be black. She was lucky to have her husband's financial support, so the whole thing was little more than an expensive hobby. She found an excuse to exit - "ill health" - although really she wasn't that ill. It was I am sure a convenient way to exit whilst saving face.
Maybe OP you could try encouraging your cousin to look at the figures carefully each month - that way she will soon see that it is futile.
As PP have said, when an otherwise intelligent person does something daft, the worst thing you can do is make them feel silly, as they will only get more entrenched. Let her come to her own conclusions, but try to find a way to ensure she reaches those conclusions sooner rather than later.