Excellent spotting that Mary Elizabeth and Margaret Ann were twins!
It's not proof, but keeping in mind that on her death cert, Margaret's son and son-in-law didn't apparently know the names of her father who was not named at all , and her mother who was named as 'Margaret Harper'. So her birthplace informed as 'New York', may also be a best guess, or a place of residence at one time, rather that a birthplace.
The fact the HULME children are enumerated as THOMPSON in the 1881 Census, mayn't be a mistake per se, it was quite common for the step-parents name to be used for the whole family. Also the younger HULME children, would barely remember their father, especially Martha, and probably Freddy. They may even have used THOMPSON in their daily lives but used their registered birth surname on official documents.
Here is a photo of the headstone for Margaret Ann THOMPSON (formerly HULME, nee HARPER.
https://www.canadianheadstones.com/view?oid=on.324861Her married daughter Mary Elizabeth & her husband William COLBORNE plus two others are also on the inscription (Colborne daughters, I think)
Mary Elizabeth and William COLBORNE married December 1880 in Brockville
I notice now, that Margaret's daughter Ellen Jane is not on the 1881 Census - and she would have been about 15 - same age as the daughter who
is on the Census named as " 'Mary (or May) 15". I think, this is actually meant to be Ellen?
and I think this, is Margaret Ann and William HULME, in 1861:
CENSUS 1861: Milton, Halton, OntarioWilliam HUME, MD, 32, married
Mrs (do) HUME , 20, married
Married in the year 1860
Both born Upper Canada
Both Religion F.C.
There could be a 15 yr old Servant, and a 18 year old male Student in their same household
Pretty sure it says MD - is that medical doctor
Cheers
AMBLY