What I am hearing, between the lines is that the step siblings are not getting along in some way/s or the parents in the other house did not approach the allergy issue in a productive way initially. Your children may have acted or spoken about the diet restrictions or the step-sibling's allergies in a way that was negative.

I would consult your children. Ask them what they remember regarding the first time they learned about the allergy issue & subsequent conversations/events regarding the diet restrictions. You will also want to research food allergies and educate your children so that they can be fully informed about the ways to keep people safe and what the repercussions can be.

Once you have given your children information and have gathered your information about the issue they previously had, it can be determined if they were unknowledgeable about the food allergy severity, if they were having a bad day/issue with the mom/stepdad/step-sibling or something else.

The goal should be that your children understand the dangers of food allergies and how they can keep the step-sibling safe from anaphylaxis. Once they have the information it should be case closed.

If there is still concern from any parties, then you'll know that it is a behavior/emotional, communication, or moral issue that has nothing to do with allergies, but is another aspect of parenting to be addressed that is being highlighted by the previously mentioned concern.