Hi Virginia and welcome to Penwith Genealogy.
I think the first thing we need to do here is to establish exactly who is who in this scenario because my initial look at the family suggests that all those aboard the Mary Shepherd were NOT of the same family.
Not only that but there was a little bit of fudging with ages - particularly with William and Elizabeth who, if I have the right family in the Census - both understated their age by a good 10 years!
Here is the list, in order of age, of those who travelled on the Mary Shepherd:-
William Northey, farm labourer age 50 (born about 1823)
Elizabeth do., age 48 (born about 1825)
Josiah do., labourer age 28 (born about 1845)
Emily do., age 25 (born about 1848)
Mary V do., servant age 21 (born about 1852)
Anne do., servant age 19 (born about 1854)
William Hy do., farm labourer age 18 (born about 1855)
Lavinia do., servant age 16 (born about 1857)
Elizabeth do., domestic servant age 14 (born about 1859)
Emily do., age 7 (born about 1866)
William Henry do., age 5 (born about 1868)
The two youngest here cannot be children of William and Elizabeth but 'might' be grandchildren.
And if you check the family in each Census from 1841 you will find that 1. there were a couple more children and 2. some from the above list did not appear with the family.
1841 Census
Seveoak, Kenwyn
William Northey, 25, miner copper, in County
Elizabeth do., 24, in County
Mary do., 5, in County
Fanny do., 2, in County
This shows that all were born in Cornwall but the later Census indicate otherwise.
1851 Census
Baldhu
William Northey, head, marr, 36, tin miner, Kea
Elizabeth do., wife, marr, 36, Plympton St Mary, Devon
Fanny do., daur, 11, Scholar, Plympton St Mary, Devon
Josiah do., son, 8, Kea
Mary Ann do., daur, 5, Kea
Martha do., daur, 2, Kenwyn
Note that Elizabeth and Fanny are now born in Devon.
1861 Census
Crofthendy, Gwennap
William Northy, head, marr, 45, miner, Gwennap
Elizabeth do., wife, marr, 45, Plympton, Devon
Fanny do., daur, unm, 20, work at mine, Plympton, Devon
Josiah do., son, unm, 17, miner, Kenwyn
Mary A do., daur, unm, 14, work at mine, Kenwyn
Martha do., daur, unm, 11, work at mine, Kenwyn
Celia A do., daur, 7, scholar, Kenwyn
William H do., son, 6, scholar, Kenwyn
Levenia do., daur, 3, Kenwyn
Elizabeth J do., daur, 1, Kenwyn
1871 Census
Goongumpus, St Day
William Northy, head, marr, 55, miner, Kenwyn
Elizabeth do., wife, marr, 55, Plympton St Mary, Devon
Martha do., daur, unm, 21, mine servant, Kenwyn
Celieenna do., daur, unm, 19, mine servant, Kenwyn
William do., son, unm, 16, miner, Kenwyn
Levenia do., daur, unm, 12, scholar, Kenwyn
Elizabeth do., daur, 10, scholar, Kenwyn
Now - let's look at the list of passengers on the Mary Shephard again but this time I will highlight those I don't think belong:-
William Northey, farm labourer age 50 (born about 1823)
Elizabeth do., age 48 (born about 1825)
Josiah do., labourer age 28 (born about 1845)
Emily do., age 25 (born about 1848)
Mary V do., servant age 21 (born about 1852)
Anne do., servant age 19 (born about 1854)
William Hy do., farm labourer age 18 (born about 1855)
Lavinia do., servant age 16 (born about 1857)
Elizabeth do., domestic servant age 14 (born about 1859)
Emily do., age 7 (born about 1866)
William Henry do., age 5 (born about 1868)
'Anne' in this list is about the right age to have been Celia Ann so I have left her in.
I may post some more thoughts in another message but for now I will leave the above for consideration.
BTW - if there is any more you can tell us about what you know of the family the please do so.
CT