FamilySearch baptised Narberth Loiusa 13 Feb 1844, and Selina 2 Mar 1845.
I'm afraid to report this is yet another instance of mis-information on the Familysearch website. There is no reference to the baptism of a Selina Thurlow at Narberth.
I did find the following in the Narberth Baptism Register;
13 Feb 1844 Louisa Mary the daughter of Edward & Selina Thurlow of Narberth a Private in the Royal Marines was baptised by Wm Lloyd the Rector.
Not wishing to confuse, but hoping this might add to your research, I gave the problem some thought(not much time lost there then) and looked elsewhere and came up with the following at the Garrison chapel ;
15 Apr 1877 William Henry the son of Henry & Sarah Jane Thurlow of the Pater Battery a Bombadier in the Coast Brigade 14th Divison Royal Aritllery baptised by Geo Wylde the Chaplin to H.M. Forces.
Turing to Lavinia Jane, I’m afraid once again there is no reference to her baptism either in the area of Narberth or Pembroke Dock.
The only possible “Lavinia Jane” that can be found is from my transcription of the parish registers of Monkton St Nicholas and that is on 6th Aug 1848 there is reference to the baptism of a Jane Lavinia, however the parents, abode and occupations are all listed as ‘Unknown’, which does not offer much solution, but best thought you are made aware.
If you think I can be of any further assistance (not much so far) let me know.
Rgds
Orielbenfro