Stevia — Food safety facts
Feed with care · Cats · full breakdown on the linked food page
The plant itself is not toxic, but many stevia blends are cut with xylitol or erythritol. Read the label.
Short answer
Focus on the cat that ingested the stevia, especially checking the product label for xylitol or erythritol. Other cats don't need intervention unless they also consumed it. Keep all artificial sweeteners sealed and away from pets. Watch the exposed cat for drooling, vomiting, or lethargy over the next 12 hours, and call your vet if these appear.