My new dog bumped my sister's mouth. Should I worry about rabies?

Fortunately, rabies in dogs is very rare in the U.S. and hopefully this dog has been vaccinated at-least once already. Knowing his vaccine status will make a big difference for knowing whether rabies is a concern or not. If he happens to have a rabies, or city registration tag on the collar, then this indicates that he would have been vaccinated before. If he is neutered, then this would also be a good indication that he has been vaccinated before.

One thing you can do in order to find out his vaccine status is to take him to a local shelter and have him scanned for a microchip. They can also help locate the owner and find out more information about the dog.

Rabies usually requires a break in the skin to be transmitted. It is also less likely that Buddy has rabies if he's not showing any signs (the vet at the shelter can check for any signs). Overall, it seems very unlikely that this dog has rabies, or that your sister would be infected.

For more information and understanding the risk of rabies, I recommend you read the information at the link below.

https://www.cdc.gov/rabies/index.html

I hope that helps and all is well with your sister and the dog.