There's also a marriage, on FamilySearch, for Helen Scandlin to William Leckie at Neilston on 31 December 1850, which I thought might be the younger daughter. However, Helen Leckie is in the 1851 census in Neilston with her husband and is living with her parents Robert and Jane Scanlon, both born Ireland. But the family may well be related to your Helen and her daughters.
Your Sarah seems to have her sister living with her in 1851, as there is a Hana Slaton (as it's been transcribed) with Arthur and Sarah Davidson. Unfortunately, as Arthur and Sarah's marriage took place before 1855 the marriage record in the OPR won't detail parents names.
If any of the sisters died in Scotland, finding a post-1855 death certificate for them on ScotlandsPeople should tell you who their parents were - or at least who the person registering the death thought the parents were.
Sadly, tracing people in Scotland back to Ireland is a notoriously difficult task, as all you tend to know is that they were born in Ireland.