Hello,
It is possible that there are two Sydenham's, though it is not a common name. However, I definitely know that he also had a girl called Sadie in the late 1850's (my auntie's can vaguely remember her, and my Grandfather went to her funeral in 1935). Sadie was also called Sarah (Ann) Just to make things confusing. In the 1911 census Sarah Ann (Sadie) was living with her older sister Eliza, who was born around 1854. This is too early for Elizabeth Peters who would have been too young, so think/guessing that he first married Mary Ann Chambers. Though think certificates are the only way.
For your interest. Helena (his last wife) was my direct descendant, with both being my Gt Grandparents.
Kind regards and thanks,
David