Ages of the Rees’s in the census records are not always consistent (!) but I think this is them below:
In 1881 the Rees family are at Plascoedca, Llannon. John (39), Mary (45), Louisa (18), Ann (10), Thomas (7), William (5), Stephen (1.5) and Nephew John Lewis (2). All born Llannon.
In 1901 Daughter Louisa, as widow Louisa Griffiths (35), is living next door to her parents at Llaethdy Cottage, with children Thomas (15), Ivor (4) and Alice (2). With Louisa’s parents at Llaethdy is grandson Isaac Griffiths age 10 (Louisa’s son).
In 1891 Louisa (25) is at Plasbach with her husband William Griffiths (25) Llanelly and their children Thomas J (3) and Isaac (1 month, who is with his grandparents in 1901), both born Llannon
This must be Isaac’s birth entry confirming her as Louisa Rees.
GRIFFITHS, ISAAC REES
GRO Reference: 1891 J Quarter in LLANELLY Volume 11A Page 1047
I think later census entries for Louisa’s birth is correct (born around 1866) and this is her birth entry
REES, LOUISA EVANS
GRO Reference: 1867 J Quarter in LLANELLY Volume 11A Page 680
So mmn Evans
However, for John and Mary Rees’s son Stephen I think this is his birth entry
REES, STEPHEN LEWIS
GRO Reference: 1879 J Quarter in LLANELLY Volume 11A Page 781
So mmn Rees.
In 1881 there is a large gap in ages between children of John and Mary Rees (Louisa (18) then Ann (10)).
My guess (I could well be wrong!) is that John Rees actually married 2 different Mary’s first a Mary Evans and then a Mary Lewi. Perhaps others can agree/disagree?
If John did marry twice and his first wife was a Mary Evans then maybe Mary Ann Evans who was adopted by John Rees was a descendant of his wife?
As an aside I think Dau Louisa later marries a George Rogers of Bath/Somerset who was lodging with John Rees and family at Llaerhdy in 1901