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.
