Could be a red herring, but just to keep track, here is "my" Benjamin Rees in 1891:
https://www.ancestry.co.uk/interacti...=successSource
The eldest son is Reginald F W Rees born 1887 Cardiff but it doesn't look as though he is Brychan Rees's father. And I think David Edgar Rees was married by 1926. Also the mother's maiden name for the Rees children is Watkins, so it doesn't look likely that these are Mary's cousins. Back to the drawing board...