A 5-year-old Congolese boy was diagnosed with the virus in Uganda, the country's Ministry of Health and WHO said.
In April an expert committee of the WHO decided that Congo's outbreak, while of "deep concern", was not yet a global health emergency.
Worldwide spread of a disease as contagious as Ebola is one of the major criteria WHO considers before declaring a situation to be a global health emergency. He sought treatment at Kagando hospital and was transferred to Bwera Ebola treatment unit, World Health Organization said. "The confirmation was made today by the #Uganda Virus Institute (UVRI)".
The health ministry revealed the measles figure when it declared the epidemic on Monday.
It was not immediately clear how the family members were able to cross the border, where millions have travellers have been screened for Ebola since the outbreak began.
The outbreak is both the second largest and second deadliest Ebola outbreak in history. Health care workers have widely deployed an effective Ebola vaccine in the affected DRC areas and intensively worked to contain cases, but their response has repeatedly been hampered by violence from the many insurgency groups, including attacks on health care workers and facilities.
"The spread of Ebola across the global border is a clear signal that the worldwide community must reset and redouble its efforts" in fighting the disease, the worldwide Rescue Committee said in a statement calling itself "extremely alarmed".