Hello crankypants,
Something does not add up.

John junior married in 1875. If he married at age 18 he was born in 1857. Ann spouse of Capt. John died in 1867 and she would have been 14 in 1857 if age at death is correct. John junior had other brothers and at least two sisters.
Marriage # 680/1874 CARDIFF THOMAS JOHNSTON MARGARET F SYDNEY
The Sydney Morning Herald Saturday 20 March 1875 p.7
MARRIAGES
CARDIFF-JOHNSTON.-July 17,1874, by the Rev. Dr. Fullerton, Mr. Thomas Cardiff, to Margaret Fulton,
eldest daughter of the late Captain John Johnston.
Marriage # 125/1876 OHLSON GEORGE RITCHARD JOHNSTON MARGARET SYDNEY
The Sydney Morning Herald - Friday 11 February 1876 p.8
MARRIAGES
OHLSON-JOHNSTON.-Jan 22, by the Rev. Thomas Gainford, George Richard Ohlson, eldest son of Captain George Ohlson, of Stepney London, to Maggie,
youngest daughter of the late Captain John Johnston, Peterhead, Scotland.
Death # 19437/1939 OHLSON MARGARET ROSS JOHN MARY WAVERLEY
Regards,
Dave