A 14-year-old girl who got into difficulty in the sea died after she became immersed in the water, an inquest opening was told.
Malaika Shamas, from Luton, was pulled from the sea at Clacton, Essex, on Thursday and died later at hospital.
She had been in the water with her brother Haider Shamas, 18, and a cousin.
Haider died on Saturday from pneumonia and brain damage brought on by drowning, Essex Coroner’s Court heard.
Essex’s coroner’s officer Jemma Cook said a lifeguard performed CPR on Malaika and she was taken to Colchester General Hospital.
A post-mortem examination recorded her provisional cause of death as immersion.
Toxicology tests were being carried out, the inquest in Chelmsford was told.
Ms Cook said Haider was also pulled from the water and was found to be in cardiac arrest. He was also taken to the hospital in Colchester.
Essex’s senior coroner Caroline Beasley-Murray adjourned the inquests until February.
She offered the court’s sympathy to the teenagers’ family “at this dreadful time”.
The siblings’ cousin, a 15-year-old girl, was also pulled from the water. She is expected to make a full recovery.
The three got into difficulty near Clacton Pier at about 13:40 BST.
Witnesses described “distressing” scenes as lifeguards and paramedics scrambled to help the three teenagers after they were spotted struggling near Marine Parade.
Several members of the public supported the rescue effort and two other people were also treated at the beach but did not need hospital treatment.
The teenagers’ father Shamas Riaz has urged parents to make sure their children can swim.
Last July, a 15-year-old boy, Ben Quartermaine, died after getting into difficulty while swimming with a friend near Clacton Pier.