To be 100% sure we have the right family ,you can order the Personal-card of Roelof Gerrit Bakker and any siblings of Roelof Johan still living or born after 1938 ( the start of the personal-card ) will be mentioned.
I assume Roelof Gerrit died in the Netherlands ?
More information :http://www.cbg.nl/download/cbg_personal_record_2013_02.pdf
You will need the date of birth of Roelof Gerrit :
He is born 1-June-1903 ( date of registration 3-June ) Steenwijk son of Roelof Bakker and Jantje Schaap.https://familysearch.org/pal:/MM9.3.1/TH-1-11352-11060-20?cc...