I'd have to disagree with your sis on this one. Minor coughs, even ones that barely affect us adults and that seem as not a big deal for toddlers can cause havoc with a small baby's still developing lungs - especially when they are younger than a few months. Most likely the baby is fine, but we have had the experience with one of my sons that he wasn't and really struggled. Better be safe than sorry - call them first thing in the morning just to get him checked out. Good luck and hope it clears up soon!
PS: My mom always reminds me part of why we pay our docs is for them to be on call when there are urgent issues and for them to help guide us in our medical decisions. I have learned not to hesitate to call them when I am in doubt - even if I wait a few hours:)