My DD was allergic to nuts. She knew she was allergic to nuts and was told never to eat them.
Her allergy (or intolerance) was not so severe, she would come out in a skin rash, which was uncomfortable but not life threatening.
When we moved house, the new school wanted her to have an allergy test before she could have school dinners.
At this point DD said that she did not think she was allergic any more as she had been eating nuts in Kindergarten. They had a walnut tree in the garden.
The point of this ramble is that she was 6yo. She knew that she should not eat nuts but when faced with the opportunity she tried some.
I do not think that it is unreasonable to expect Keresely's inlaws not to leave food with nuts within her DS's reach. Especially sweets with nuts.
If it were your 4yo, would you take the chance that he would not be tempted?
If it were my son, I would stop contact with the inlaws. If they think that their enjoyment of nuts is more important than the health of their grandson's life, then they do not deserve to have contact.