Possible explanation:
The 1871 census has Margaret Kewley aged 45, suggesting that she was born 1825/6.
The 1861 has her aged 38.
In 1851 William Kewley and Margaret Christian (age 25) are visitors at the Quay abode.
There is a Braddan marriage on April 24 1851 (three weeks after the census) between Margaret Craine and William Kewley.
No mother's surnames are given in the IGI christening records for William and Margaret's children.
It seems possible that Margaret was born prior to the October 1829 marriage of John and Lucy, and was christened after their marriage, possibly a second christening - the first being as Craine. This does not explain who Margaret Christian was, unless her natural father was named Christian and she went by that name, and may have been christened under that name initially.