Hi Bitzar
I’ve had a look at the various records and indexes I have, but unfortunately haven’t found anything conclusive about your Ninianson ladies. The ages of Elizabeth and Issabella in the 1841 census (with parents William and Isabella) do match the women who went to Australia. They appear in the 1851 census as Elizabeth Ninionson, 16, and Isabella Ninionson, 12, living in Sandsting and Aithsting parish – the same parish as the family in 1841. Neither girl is with her parents, so it isn’t possible to say if they are the same ones, but I think it is likely. It may be that they later went to Australia – to rule them out would require searching forward to see if there were marriage or death records for them in Scotland. The 15-year old Gean Ninianson in the 1841 Census could certainly be Jean or Jane. (Adult ages were "rounded down" to the nearest 5 years in 1841 so Gean may have been a little older than 15.)
I haven’t found baptisms that match any of the women. The only possible is an Elizabeth Ninian baptised on the island of Yell in 1832, but her father was Gilbert, not William. Of course not all baptisms were recorded, many church registers have been lost or destroyed, and families could have belonged to other denominations of the church whose records may not be online.
Don’t get hung up on different spellings of the surname. At that time there was no “correct” spelling and people wrote names as they heard them (accents made a difference). If you’re looking at modern transcriptions, remember that people often have difficulty reading old handwriting, especially if it’s a name they’ve never come across before.
I’ll keep a look out for the Niniansons in future research and let you know if I find anything relevant.
Lizci