This sibling that i mention, Alice, is the oldest in the family
Might she have been born before her parents' marriage?
Just following up on this thought - I see she is Alice Fisher.
Parents Edward James Fisher and Martha Clark married at St Anne, Wandsworth on 18 July 1859.
Birth reg for Alice Fisher mmn Clark is Jun qtr 1860 Wandsworth, and she is 11 months old in the 1861 census (7 Apr 1861). This is more or less consistent with the birthdate of 9 May 1860 recorded in the 1886 baptism register.
So not, it seems, born before the marriage (or even on the way at the time).