As you are unable to find the death of James COSTIN(S) and although Hannah COSTINS death is registered 1853 - Cheltenham - no entry on Ancestry, Gloucestershire burials for Cheltenham - did they become Non-conformists

Forest of Dean Family History Records
Show baptisms
1872 Hannah Maria COSTINS - parents: James & Maria : Monmouth Methodist
1874: Jos James COSTINS - parent Jos James & Maria : (Non-conformists records)
1877 Thomas Charles COSTINS - parents: James & Maria : Coleford Congregational
James COSTINS married Maria COWLES - 1872 Monmouth District.
1881 Census, Glos St Nicholas
Maria COSTINS, Widow - 28 - Washerwoman - born Churcham, Glos
Julia COSTINS - 11 - born Monmouth
Hannah COSTINS - 8 - born Monmouth
Joseph J COSTINS - 6 - born Glos. Coleford
Thomas C COSTINS - 4 - born Glos. Coleford