I can see that being the answer IF she hadn't specifically invited them to attend. Like if she had only invited Simon's parents but they showed up with the other 2 as well. Of course, this wouldn't ever happen and even if it did, the dish she had made wasn't one that would easily accommodate two unexpected additional guests. And even if she somehow could manage it, there would have been no reason why she would have served them poisoned food as well. So this is all moot, because for all practical purposes, it wouldn't happen. But I'm saying collateral damage might make sense to explain it if it did, but it doesn't make any sense at all when we know that she specifically invited them along with the Pattersons.
She didn't HAVE to invite them. I doubt she had any reason to think that the Pattersons would only come if the Wilkinsons did too. They were, AFAICS, a completely superfluous addition to the luncheon guest list. So I really wonder what was in her mind when she asked them. Seems like she must have had some real reason in her mind to include them. But she could even have invited them but not served them anything harmful! I'll probably never understand it.
In general, I find it easier as the host to have at least two parties who can chat with each other if I'm occupied with the food.