It is recommended that all immediate family members be tested regardless of whether they have symptoms or not, because having a family member with the disease is the strongest risk factor for disease development. (HLA DQ positivity is the second strongest risk factor) The risk that another family member also has coeliac disease is about 10%. Not uncommonly, family members may have nonspecific symptoms such as irritable bowel syndrome which might actually be due to coeliac disease.
If the tests come back negative, this only tells you that they are unlikely to currently have coeliac disease, but they could still develop it in the future, and may require retesting. Infants who have not been introduced to solids, and young children need not be tested until they are older (approx.) three to four years of age, unless they have symptoms.