Unfortunately the 1939 Register doesn't give family relationships. Given where he was born, there's also a reasonable chance that Cyril was an evacuee at the time the register was compiled - some children went to relatives but many were living with strangers. The couple he was with were aged around 50 in 1939, and the two redacted records at the same address will be for other children who, from what you say won't be his siblings, as he didn't have any.