I find the couple in Drouin records, 5 stillborn children one after the other from 1834 to 1843, after a daughter Marie Adeline born in 1831, all in St-Hughes, the mother is listed as Catherine Sey on those records.
On the marriage of their daughter Marie Anne Withman (sic) to Paul Ménard in 1878, Catherine is listed under the name Sed. She's a widow, John Withman is listed as deceased. Resident of St-Hughes.
I find the marriage of John Weightman to Phoebe Mary Hungerford on 18 May 1843 in Stanbridge (Baptist church), he is listed as 21. No parents named, so no idea if this is your couple's son.
Looking at the 1861 census, your Catherine's name looks like Segnets or Seynets, that is definitely an S at the beginning. Note that she is listed as born in Bas-Canada on that census, whereas her husband is listed as born in England. Their daughter Ann is with them, aged 30.
When I look at the 1871 census you have on her profile, page 2 line 4 has her as Catherine Witman age 70, church of England, a widow. Her origin is listed as ''Allemande'', ie German.
1881 census pg 2 line 8 has her listed Catherine Witman, aged 95, born in USA.
Can't find them on 1851 census, but with name variations abounding for her husband, nor surprising.
So, would suggest you look in immigration records if you can, or ship lists. They don't appear to get married here. Of course, if she was born in USA, then she won't appear in immigration records, since the border was truly open back then, no records exist, nobody kept track.