The Bucks County Courier Times (http://bit.ly/1bYO05P) said the baby was sleeping when the incident occurred around 9:30 p.m. Tuesday.
Bristol police Sgt. Joe Moors said family members were baby-sitting for the girl's parents when two relatives started fighting and one grabbed a hammer.
Moors said a third family member snatched the hammer and tossed it behind his back. It struck the sleeping baby in her right eye, causing severe injuries.
Police said doctors at Children's Hospital of Philadelphia are assessing the damage.
Moors said the injury was accidental but the incident remains under investigation.
Information from: Bucks County Courier Times, http://www.phillyburbs.com