Haider Shamas died on Saturday - two days after his 14-year-old sister Malika lost her life in the same incident.
Another relative, a 15-year-old girl who also got into difficulty whilst swimming, is expected to make a full recovery.
Lifeguards and paramedics scrambled to help the three teenagers after they were spotted struggling near Marine Parade at about 1.40pm on Thursday.
Acting Detective Superintendent Paul Wells said the three teenagers were recovered from the water close to Clacton Pier.
Members of the public joined the rescue and two other people were also treated at the beach but did not need hospital treatment.
All three were taken to Colchester General Hospital.
Witness Becky Bryant said: "It was so distressing, everyone was trying to do what they could - lifeguards, air ambulance, they truly worked as a team."
She added that the teenagers were part of a group of up to 30 people at the beach.
The East of England Ambulance Service said its first responder arrived at the incident less than seven minutes after it was alerted to the incident.
Floral tributes have been left non the railings by the beach.