Cough is a common concern for parents. It has both minor as well as serious causes. I encourage you to talk to your children's pediatrician
and seek evaluation. While hopefully minor, there are serious conditions that deserve to be ruled out.
Firstly, cough is a common reflex. Humans have developed the cough reflex to clear the airways to allow for proper ventilation of the lungs. An occasional cough is expected to clear normal mucus that may develop. There is no frequency of cough that is normal range, but if you think your kids cough 'a lot' that is a concern.
There are a few causes of cough that should be ruled out. Firstly, one should rule out an infection. While unlikely a cause of chronic cough, many viral and bacterial infections are possible in kids. Fevers or a productive cough is worrisome. Allergies is another common cause in kids. Specifically, sisters in the same house are at risk for being allergic to as well as exposed to the same allergens. Common ones include dust mites (like on pillows) or pollen. Another common cause that should be ruled out is asthma
. Asthma often causes wheezing and shortness of breath as well--but cough can be the only symptom.
I would encourage evaluation by your pediatrician as a lot of coughing is not normal.