The source of almost all the basic data for Scotland is Scotland's People
www.scotlandspeople.gov.uk or SP for short. It's pay-per-view but modestly priced.
SP has two marriages of a John Campbell to a Christina/Christian Sinclair, one in Bowmore and Kilarrow (Isle of Islay) in 1825 and one in Leith in 1826. Note that this doesn't mean there wasn't another one of which no record has survived.
John Campbell and Christian Sinclair had several children baptised in Bowmore and Kilarrow between 1826 and 1830, so I think you can safely eliminate them from your search.
The index listing at FamilySearch for the Leith marriage also mentions an Angus Sinclair. You would need to view the original at SP to see what it says and why it mentions Angus Sinclair.
One of the useful tools in finding Scottish ancestors can be the naming tradition -
First daughter after mother's mother
Second daughter after father's mother
Third daughter after mother
First son after father's father
Second son after mother's father
Third son after father
Subsequent children after great-grandparents, uncles/aunts, other relatives, friends, minister/doctor/laird/schoolmaster/prominent citizen or their wives.
But I don't think it will be useful in this family.
I see from the 1841 and 1851 censuses that the children were Christiana, Amelia, Catherine, James, John, William and Alexander, and that there is quite a long gap between Christiana and Amelia, so there could have been two or three missing children. If there were, and we don't know their names, we can't deduce anything from their names. The only possible inference would be that Christiana's mother's name was Christiana or a variant thereof.
It also doesn't help that the sons are James, John, William and Alexander, those being by far the commonest male given names in Scotland.