There is an 1851 census for some provinces (BC is not one of them), but this is not nationally indexed, so you'd need to know WHERE in Canada they came from.As Charleston was a reasonably large immigrant port, what information makes you believe the family would have come through Canada? It seems a rather roundabout way to travel.Have you checked the Charleston passenger lists for your relatives?
If any relatives were born in Canada, check census records for notes like 'Canada English' 'Canada French' etc as these generally mean that the person was b. in Ontario / Quebec respectively.
You might also want to see if there's any reference in family records (birth/marriage/death, wills, etc) to other relatives of Alexander and Isabella, like siblings who might have immigrated with them.
Did Alexander become a US citizen? Although early naturalization records often don't contain much information, they will give you an idea of when he came to the US as there were length of residency requirements before you could become a citizen.