Having looked on the 1939 register on Ancestry, I noticed that 2 Tanyfron Terrace bad the name 2 Bryn Tegai.
Living at 2 Bryn Tegai on the 1939 register is a Hugh Roberts born 17th Dec 1884, Unemployed and Married. Also- Jane M Roberts born 13th Aug 1887, Unpaid domestic, Married. Also- Thomas Roberts born 1st April 1927, at school.
Could this ‘Jane M Roberts’ be the sibling’s of your William Roberts?