Find Ancestors

I am trying to find more information on my grandfather James Youens Martin. I have found a few on an ancestry site and have even obtained a birth certificate but I am still not convinced I have the right person. His father also bore the name James Youens Martin. When my grandfather married Florence Lilian Osborne in 1921 he resided in Morant Street Poplar. (They had 1 daugher Florence in 1923. She later married Thomas Santola Fletcher in 1943). The birth certificate I have shows that he was born in Gravesend Kent in 1896.. His mother stated as Annie Marie Shovlar from Kent. It does all fit but his siblings May, Dolly, Gert, Dick and Frances are cauising a blockage. I cannot find any birth records on these owing to the nicknames and lack of knowledge as to where they were born. Later in life Gert was in the army and at some point married a Sid. May married William Hayes and they had 2 or 3 girls, Vera, Joan and I believe Ellen. Dolly never married or married after her son Harry was born. He still kept the name Martin but she took the name Scott. Her spouse or partner went by the nickname Scottie. He died during the war I believe. Dick was in the Navy and died a young man. I have nothing further on him and Frances remained a Spinster. Dolly, May & Frances ended up in Upminster Bridge Upminster Essex. Although I feel I am doing very well here, I want to go back into the lives of my great grandparents so if anyone can help I would be grateful.