presuming is a dangerous thing to do with family history. The Catherine Morris you refer to on the 1841 census is in a separate household within Mount Pleasant and is enumerated as a servant which is a little unlikely for a sister of Lewis. The 2 Williams are also in this separate household and servants. There is a / to show the end of household next to John's name
http://www.users.waitrose.com/~census/1841_transcription_hints.htmRebecca is a sister and regular visitor by the looks of it - or at least at census time
1851 Mount Pleasant HO107/2472 folio 134 pg 1
Lewis Morris head mar 49 attorney at law b. Cards, Aberystwyth
Sophia wife 45 b. town of Carmarthen
Rebecca sister 50 b. Aberystwyth
William son 13 b. Carmarthen
Charles Edward son 9 b. Mount Pleasant in Llangunnor
John son 11 b. Carmarthen
Margaret Thomas 57 general servant b. Kidwelly
Jane Thomas 22 general servant b. Kidwelly
Ann Thomas 22 genreal sedrvant b. Carmarthen
1861 Mount Pleasant RG9/4134 folio 12 pg 18
Lewis Morris head mar 61 Solicitor Registrar(?) of County Court of Glamorgan (at Swansea) b. Cards, Llanbadarn fawr
Sophia wife 52 b. Carmarthen St Peter
William H son 23 articled clerk b. Carmarthen St Peter
John son 22 undergrad Jesus College Oxford b. Carmarthen St Peter
Rebecca visitor 63 private property b. Llanbadarnfawr
Anne Jones 50 servant b. Carms, Mydrim
Jane Thomas 30 servant b. St Peters
Elizabeth Woolf 18 servant b. St Peters
Harriet Harries 40 mantua maker b. St Peters
The IGI batches for Llanbadarnfawr and Aberystwyth don't go back far enough to look for baptisms of the Morris family. The parish records are held in the record office and the National Library in Aberystwyth. Pity I didn't know a couple of months ago when I was in the library searching again, and in vain, for my Thomas family.
Lewis Morris married Sophia Hughes at Carmarthen St Peter's 25/10/1830
children of the couple appear in IGI batch C059251 for Carmarthen St Peter
Lewis Morris born 6/12/1831, bp 4/1/1832, died 21/4/1832
Lewis born 23/1/1833 bp 25/2/1833
Sophia bp 20/3/1835 died 26/5/1836
William Hughes bp 4/1/1837
John bp 17/5/1839
Charles Edward bp 1/10/1841