Police say the incident happened Thursday at a home in St. Clair Township, Westmoreland County.
Investigators say the teens recorded the incident and posted it to Snapchat.
The 18-year-old and 17-year-old girl could face child endangerment charges.
Police say other than the coughing, the child did not display any other visible effects of inhaling from the device.
The girls' names have not been released.