I am trying to make a point to someone that just because two plants share a family and one plant is safe for human consumption, it does not follow that the other plant also is safe for human consumption. Can anyone provide an example I can use as proof?
The most classic example if you want to win this argument would be the family Solanaceae.
Also referred to as the Nightshade family, it includes the deadly nightshade or Atropa belladonna and many other plants not safe to eat.
Other members of the family are tomatoes, peppers, potatoes, and more.
Plant families can be massively diverse, and toxicity doesn't really have much relationship to family. Most of the compounds that are found in plants that are toxic are found in other non-toxic plants as well: dose is crucial.
Fungi are not plants and you've tagged this as botany, so this is perhaps off-topic, but I feel like it might help you make your point: the genus Amanita contains extremely toxic species (A. phalloides), highly regarded edible ones (A. caesarea) as well as psychoactive ones (A. muscaria).
While all the other answers have described one plant family having both edible species as well as poisonous species, I am compiling all the families in one answer.
Mangos (Mangifera indica) and Cashews (Anacardium occidentale) belong to Anacardiaceae, and also the poisonous Sumacs (Rhus spp.).
Carrots (Daucus carrota), Parsnips (Pastinica sativa), Dill (Anethum graveolens) and the poisonous Poison Hemlock (Conium maculatum), and Water Hemlock (Cicuta spp.)
Milkweed (Asclepias spp.) and the poisonous Dogbane (Apocynum spp.) is commonly known as the poisonous relative of Milkweed.
Marsh Marigold (Caltha palustrus) and the poisonous Buttercups (Ranunculus spp.)
Food crops like Potatoes and Tomatoes and deadly poisons like Deadly Nightshade (Atropa belladonna), Jimson Weed (Datura spp.) etc.