“Twins Catherine & Bernard were born in 1933 (know this from family and also found records on familysearch.org)”
Unfortunately, the familysearch.org research has proven to be less reliable than the UK BMD Index search for this family. It certainly appears that there were just 4 children born to Thomas Frail and Helen Hughes during the time period given. These were: Thomas (1932), Peter H (1936), Irene (1938), Vincent (1939).
Catherine, born Q4 1932, died Q1 1934, before her 2nd birthday and is the daughter of Peter Frail and Mary Sheils (who married Q4 1926).
There are 2 options for Bernard, but neither associates him with Frail/Hughes.
Bernard 1, born Q3 1932, died Q1 1935 before his 3rd birthday and is the son of Bernard Frail and Margaret Little (who married Q4 1931).
Bernard 2, born Q3 1931 is the son of Leonard Frail and Doris Brockley (who married Q1 1922).
Peter Frail, Catherine’s father, and Bernard Frail, Bernard 1’s father, appear to be Thomas Frails younger brothers as all can be found in the 1911 UK census at No. 21 Heslops Yd, Birtley, Durham.