A number of shots were fired through the kitchen window of their home in the Market district of the city centre.
The gun attack happened at Eliza Street Terrace just before 9 pm.
One bullet struck the floor close to where the 15 -year-old was sitting.
A second round hit the ceiling above the grandmother had been sitting just seconds earlier.
It is the second time the house has been attacked and maybe be linked to a feud in the area.
Crime scene investigators spent the day combing the area for shelling cases and possible clues to the gunman.
Detective Sergeant Natalie McNally of the Reactive and Organised Crime Branch said: “I would ask anyone with any information about this incident to contact detectives at Musgrave Police Station on the non-emergency number 101.
“Or if someone would prefer to provide information without giving their details, they can contact the independent charity Crimestoppers and speak to them anonymously on 0800 555 111.”