what a horrific situation! But they all seem to be being really ignorant about this. jeez my mum is allergic to asprin, so it was banned in all ways shapes and forms, together with ibuprofen. Do you think anyone kicked up a fuss? No! And pain killers are more flaming important than a dish of sweets or a cereal!
Perhaps a short term break from them, to let it all cool down a bit.
Then yet another conversation to explain ds condition one more last time, and how you can't possibly have nuts in any proximity to him. Full stop.
Suggest you say you've taken a huge amount of impartial advice, and it is not unreasonable for family members to forego something so incidental for the sake of a childs life......
Until such time as they are willing to place their grandson above a bowl of sweets, or a box of crunchy nut, you will not be seeing them unless on nut-free territory. They are however welcome to come visit you whenever they wish, but on strict understanding that nuts are not brought.
Wishing you all the best! Oh and don't blame yourselves for any of this, they seem to be in a world of their own, selfish and silly. Just cos you're related, doesn't make them good examples to be around.