I can understand you wanting to protect your CAH child.  But I have to ask why you agreed to look after your nephew when you know that children get sick, coughs, colds etc.   You should not have put yourself in this position.  A simple explanation in the beginning that your CAH child comes first and that you would not be able to care for your sister-in-law’s child any time he was sick, and that it would be better to put him in day care or find alternative arrangements.  You haven’t said how old your CAH daughter is, but also keep in mind that protecting her too much will not allow her immune system to build and when she does come in contact with other children and environments (school/playgrounds) she will most likely pick up all the illnesses going around.