Three other members of the family are suspected to have recently died from the virus.
The cases were reported in the southern region of Nzerekore, where the outbreak began in December 2013.
The Ebola outbreak killed more than 11,300 people - mostly in Guinea and its neighbours Sierra Leone and Liberia.
The new cases were reported by the World Health Organization (WHO) hours after it declared the latest Ebola flare-up to be over in Sierra Leone.
The WHO has warned that sporadic cases of Ebola are likely to re-emerge as the virus can linger on in body fluids of some survivors.
Analysis: Tulip Mazumdar, BBC global health correspondent
This is yet another blow in the long lingering fight against Ebola. But it is not unexpected.
Guinea was in fact the only one of the three worst affected countries that hadn't had a re-emergence of the virus after the outbreak was officially declared over there on 29 December 2015.
Both Sierra Leone and Liberia have reported little clusters of new cases after declaring the outbreaks over. But they've been dealt with quickly.
A risk of new flare-ups remains because Ebola can persist in body fluids of some survivors for months after they recover.
A very small number of new cases have been linked to sexual transmission.
The world is in new territory here - scientists are still learning as the worst Ebola outbreak in history continues to unfold.
More than 17,000 Ebola survivors are dealing with a wide range of complications and social stigma.
Some scientists say there's a risk the virus may become an ever-present disease in West African society.
11,315
Deaths - probable, confirmed and suspected
(Includes one in the US and six in Mali)
4,809 Liberia
3,955 Sierra Leone
2,536 Guinea
8 Nigeria
Two new Ebola cases have been confirmed in Guinea, almost three months after it celebrated the end of the outbreak.