Appreciating that some of this is best guess, this is where I have got to on the timeline now:
It seems to work out timewise. Now need to get the evidence.
Cassie Elias timeline
1852 - Thomas Elias (Cassie's father) born in Bedwelty, Monmouthshire
1854 - Mary Hannah Harris (Cassie's mother) is born.
1855 - Cassie's future step mother Emma Matthews born in Rudry, Glamorgan
1861 - father Thomas aged 6 living with family in Rumney near Tredegar
1871 - father Thomas (20) visiting family of Mary Hannah Harris (16)
1873 - Thomas Elias (22) marries Mary Hannah Harris (18) at Eglwysilan on 31st May.
1874 - sister Hannah is born
1875 - Cassie is born
1875 - Elizabeth Ann Matthews (Cassie's future step sister) born in Machen, Monmouthshire
1877 - Isaac Matthews (Cassie's future step brother) born in Machen, Monmouthshire.
1878 - birth of sister Mary Ann
1879 - death of sister Mary Ann (aged 0)
1880 - Cassie travels to America with her mother Mary and older sister Hannah.
1881 - possible death of sister Hannah in America. (No records found to prove this yet).
1881 - probable return to Wales (no passenger records found so far)
1882 - Cassie' s mother dies in Crickhowell when Cassie is 7 years old.
1887 - Cassie's father Thomas (35) marries second wife Emma Matthews (33) in Pontypridd.
1891 - Cassie (16) living at and working as a barmaid at the Castle Hotel, Caerphilly
1891 - Thomas (39) and Emma (36) Living in ... with their children.(Elizabeth Ann Elias 16 and Isaac Elias 14).
1896 - Cassie marries Hiram Aston in Bedwelty, Monmouthshire