I think the Morgan connection is through the Edwards line:
Maryam Edwards with the Llewelyn family in 1871, is I believe the sister of Mary Llewelyn (nee Edwards). She also appears in 1871 with her parents:
26 Marwell St, Ystrad (Home)
Edward Edwards Head age 62 born Ystrad Collier
Ann Edwards Wife age 49 born Amlwch, Anglesey Collier's Wife
Merriam Edwards Dau age 17 born Aberdare Servant
Gwenllian Edwards Dau age 15 born Aberdare Servant
Edward Edwards Son age 13 born Aberdare
John Edwards Son age 11 born Aberdare
David Edwards Son age 5 born Aberdare
RG10 Piece 5386/40 Pg 13
This Ann Edwards is the same person that Mary Ann Llewelyn is staying with in 1881.
In 1861, Gwenllian and Miriam are with parents Edward and Ann, plus other children were at 12 Oak St, Fforchaman, Aberdare. Also Edward's 91 year old mother Mary Edwards.
RG9 Piece 4065/98 Pg 46
In 1851, as well as Gwenllian, there is a Mary Edwards age 7, Club? Houses, Forchaman, Aberdare,
HO107 Piece 2460/241 Pg 23
There's a marriage of Gwenllian Edwards and John Morgans in 1875
Pontypridd 11a 385
and a marriage of Mary Edwards and Lewis Llewellyn in 1863
so if this is the right connection, Gwenllian and Mary were sisters, and Mary Ann Llewelyn was a cousin of John Edward Morgan.
What do you think?