I can't find a tree that helps.
NSW Birth 12505/1866 GILES CATHERINE A - JOHN - SARAH - ORANGE * just 1866
NSW Marriage 6303/1893 COTTAM SETH H - GILES CATHERINE A - REDFERN
The Sydney Morning Herald - Wed 24 Jan 1894 p.1
COTTAM - GILES.-January 10, at St. Matthias' Church, Paddington, by the Rev. J. W. Gillett, B.A., Seth H., youngest son of Thomas Cottam, contractor, Redfern, to Catherine Anne, fourth daughter of John Giles, builder, Paddington.
Three older sisters born in NSW ??
GILES MARY A 4697/1860 JOHN - SARAH - BATHURST
GILES SARAH J 9866/1862 JOHN - SARAH - FORBES
GILES ELIZABETH E 15984/1864 JOHN - SARAH - WELLINGTON