Mortar rounds hit two houses in villages outside the provincial capital El-Arish Wednesday night, killing mostly women and children.
Separately, an explosive detonated as a military vehicle drove past, killing two policemen, police said. Officials have not said who is responsible.
The government is fighting Islamist militants in the region.
Hundreds of soldiers and police have died in targeted attacks in the Sinai peninsula since the military overthrew Islamist President Mohammed Morsi in 2013.
Last month, a suicide bomber targeted a police compound in El-Arish, killing himself and a civilian.
Sinai Province, a jihadist group affiliated to Islamic State, claimed it was behind the attack, local media reported.
The group, which was known as Ansar Beit al-Maqdis until it pledged allegiance to Islamic State leader Abu Bakr al-Baghdadi in November, has claimed responsibility for most of the major attacks in Sinai.
The government put North Sinai under a state of emergency and a curfew after a checkpoint attack in October that killed dozens of soldiers.
Eleven civilians and two policemen have been killed in attacks in Egypt's restive North Sinai, officials say.