First, I would trying putting in a carbon monoxide detector in your house, those symptoms (fatigue, headache) sound like it might be the culrpit, it's a must for any place you live anyway.
Otherwise, look to what has been introduced to the air, inside or outside. If your house has an attic, make sure you don't have a bat infestation. The ammonia from their droppings can affect you.
Good luck, I hope you find what's affecting your family.
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