You don't appear to have any birth certificate, for either Charles or Maria?
The 1881 ref is RG11/760; F72; P5 (epsom, surrey).
1891 ref? RG12/650; F57; P52 (northfleet, kent)
1901 ref? RG13/716; F14; P20
Do the school hold any helpful records?, What were the criteria for admission?
what parental information is given on the 1890 marriage certificate?
How have you deduced that Maria and Charles are in fact siblings?
Pauline