As aghadowey has said, 14 was just a household number, that is, the 14th household the enumerator visited in that townland rather than a number allocated to the property itself that remained with it always.
Margaret McKeown may or may not have led the way for her sisters. To be sure, you would need to find the record of the arrival for the Margaret McKeown you found previously and work through the information gleaned to rule in/out it being her.
It is not that unusual for one member of a family to be present at the death of a parent and sign as witness on the certificate whilst a different member is present and signs at death of other parent, even if not eldest.
As you have had contact previously with St Mary's, did you ask them to check in case Arthur and Sarah married there in the 1850s - it would seem marriage records for the church exist from 1848 - 1880 (albeit if Sarah was from another church, it is more likely they married in that)?