I have the same problem relating to my 2 x g.grandfather although he was born in 1834 so there won't be a birth certificate. I found the baptisms (I actually visited the archives and saw the actual parish registers) of his older siblings - including 2 older sisters I didn't know about - and also his younger brother, but nothing for my 2 x g.grandfather. My feeling is that his eldest "sister" was his mother and also the mother of his youngest brother. Unfortunately, she died soon after the youngest child was born and the youngest one was baptised as though my 3 x g.grandparents were his parents. However, on the 1851 census, he was living with his surviving "sister" but listed as nephew!!
Perhaps something similar happened with your g.grandfather