Airbag hit my sisters chest (breasts) in a car accident a few days ago. She didn't tell the drs because it wasnt bad but theyre covered in red bruises?

Take her back to . urgent care to get looked at for burns and possible MRi/cat scan to asses for internal injuries. Find out what is going on to stop it and treat it, its easier that way. Delaying help makes things worse. .
See doctor. That happens occasionally after airbag deployment. It may be nothing but it's still important to get it checked out to make sure there isn't any soft tissue damage in the chest (breast, muscle, lungs, heart) and if not, make sure it's just bruising. It's also important to get early medical evaluation in case she ever needs to file a claim with the auto insurance.