Hookworm infection: Despite the negative stool culture, hookworm infection remains a strong candidate due to the presentation of iron deficiency anemia in multiple family members. Hookworms can cause chronic blood loss leading to iron deficiency anemia, and their presence can also contribute to anemia of inflammation. Chronic constipation could be related to the infection, although it's more commonly associated with diarrhea. A weakened immune system could be a consequence of chronic infection and malnutrition.

Celiac disease: This autoimmune disorder can lead to malabsorption of iron and other nutrients, resulting in iron deficiency anemia. It could also explain the overlapping anemia of inflammation and might contribute to constipation and immune system issues if left untreated.
Inflammatory bowel disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause chronic inflammation, leading to anemia of inflammation, iron deficiency anemia due to blood loss, and constipation or diarrhea. They can also affect the immune system.
Chronic blood loss from another source: Other sources of chronic blood loss, such as gastrointestinal ulcers or cancer, could explain the iron deficiency anemia but might not directly relate to constipation or a weakened immune system unless they lead to significant malnutrition or chronic disease.

Gastric cancer: Although less common, gastric cancer can cause iron deficiency anemia due to chronic blood loss and could potentially lead to constipation changes and a weakened immune system due to malnutrition and the cancer itself.
Colon cancer: Similar to gastric cancer, colon cancer can cause iron deficiency anemia through blood loss and could lead to changes in bowel habits, including constipation, and impact the immune system.

Hereditary hemorrhagic telangiectasia: A rare genetic disorder that leads to the formation of abnormal blood vessels, which can cause chronic blood loss and iron deficiency anemia. It might not directly explain constipation or a weakened immune system unless there's significant malnutrition or organ involvement.
Rare parasitic infections other than hookworm: Other parasitic infections, such as those caused by tapeworms, could potentially lead to similar symptoms, although they are less common and might have different stool test results.

A CBC (Complete Blood Count) with a hookworm infection might show:

Low hemoglobin and hematocrit levels, indicating anemia.
Microcytic, hypochromic anemia on the differential count, characteristic of iron deficiency anemia.
Possibly elevated inflammatory markers, such as an elevated white blood cell count or platelet count, if there's an associated inflammatory response.
Low mean corpuscular volume (MCV) and mean corpuscular hemoglobin (MCH) values, further supporting microcytic, hypochromic anemia.