From the way Stanley worded his dinnertime announcement, it appeared he had already told Ellen of his intentions, since he spoke specifically to his children.
It dawned on Briggs that Stanley’s announcement took his children by surprise. It was hardly likely one of them would just happen to have poison handy at the holiday dinner to use in case something like this occurred. The probability of one of them being the killer was very low.
Ellen, after being told of Stanley’s plans, had visited the chemical plant, got the poison, and put a few drops into Stanley’s crystal drinking glass. That way Stanley, and only Stanley, would be poisoned as the entire family helped themselves to food and drink.