Introducing Allergens When a Sibling Has a Food Allergy

The 2026 ASCIA guideline on infant feeding for food allergy prevention names this group directly. It is aimed at all infants, it says, including those at increased risk such as "infants with siblings or parents with allergic conditions". The recommendations that follow do not set them apart. ASCIA "recommends timely introduction of egg for infants irrespective of their family or personal history of allergy", and says the same of peanut.

The NIAID addendum guidelines for peanut allergy prevention from 2017 sort babies into three groups, and each is defined by the baby's own skin and history: severe eczema, egg allergy or both in the first, mild to moderate eczema in the second, neither in the third. A brother or sister's allergy is not one of the criteria. The NHS page on food allergies in babies sounds more cautious: with a family history of food allergies, eczema, asthma or hay fever, "you may need to be particularly careful when introducing foods. Talk to your GP or health visitor first." We read those as fitting together. The plan usually stays the same, and the conversation with the doctor comes first.

The largest study we found on this comes from Chicago. Gupta and colleagues (Journal of Allergy and Clinical Immunology: In Practice, 2016) looked at the brothers and sisters of 1,120 children with a food allergy. A third of the siblings, 33.4%, had no sensitisation and no symptoms. Fifty-three percent had a positive blood or skin prick test to a food but no reported symptoms of food allergy. Only 13.6% were both sensitised and reacted to the same food. Milk allergy was the most common among the siblings at 5.9%, then egg at 4.4% and peanut at 3.7%.

The authors concluded that testing siblings "without a history of clinical reactivity appears to be unjustified", and that screening "may lead to negative consequences related to potential misdiagnosis and unnecessary avoidance of a food." Put in plain numbers, for every sibling in that cohort with a true food allergy, roughly four had a test that looked positive and ate the food without trouble.

A positive skin prick or blood test means the immune system has made antibodies to a food. It does not mean eating that food will cause a reaction, which is the gap our post on allergy testing for babies goes into. For a baby who has never eaten egg or peanut, a result like that can turn into months of keeping the food out of reach while the family waits for a specialist.

That wait has a cost of its own. The NHS page says that "delaying the introduction of peanut and hen's eggs beyond 6 to 12 months may increase the risk of developing an allergy to these foods." A test ordered to protect the baby can end up holding back the very foods the guidelines want in the diet early. None of this means testing is never right. A doctor may still want it for a baby with severe eczema or a reaction already on record, and that is a decision they make with you.

This is the part the guidelines spend the least time on and families spend the most. If an older child is allergic to peanut, the house may have been peanut-free for years. The baby, meanwhile, is meant to keep eating it. The ASCIA guideline says that once a common allergen has been introduced, and unless the baby has reacted to it, it should be offered at least once a week. Our post on keeping allergens in the diet covers why that matters.

We have no tidy answer for this, and it belongs with the allergist who looks after the older child. Ideas families raise with their doctors include serving the food while the older child is out, keeping a separate bowl and spoon that never go near the other child's plate, and washing the baby's hands and face and wiping the high chair straight after. Whatever the plan, the form matters for the baby as well: smooth peanut butter thinned into porridge, never whole nuts, which are a choking hazard at this age.

In our house the toddler loves sharing, and the baby takes whatever is offered. With an allergy in the family that habit matters more, because a crust passed across the table counts as an exposure whether anyone planned it or not. We noted those moments in the baby's log next to the planned meals.

The other habit that paid off was keeping each child's history apart. A family history is a fact about the family. What the baby has eaten, how much, and what happened afterwards is a fact about the baby, and it is the record a doctor uses to decide whether a test or a supervised first feed is worth it. Our post on solids with more than one child covers the day-to-day side. Hives, swelling around the lips or eyes, or vomiting soon after a new food is a reason to stop and call the doctor. Trouble breathing, floppiness or a swelling face is an emergency call, not a log entry.