Cough on babies!

My two month old has a bad cough and it seems to me like her throat hurts aswell. She eats normal and everything but her cough doesnt sound good! Her older sister has step throat and i feel she might have gace it to the little one. I already took her to her pediatrician and would not give my baby and medicine. What is there to do now? I used the saline drops and all but nothing helps 😓😓😓