I thought I will try and find Catherine's sister, Margaret Sinclair. According to the 1881 English census birth date is 1808 Scotland.
The original 1881 census does
not state a year of birth (the UK census never gives this information). It says that she was 73 years old. As the census was taken on 3 April 1881, and assuming that her age is accurate, she would have been born between 4 April 1807 and 3 April 1808.
I
wish these web sites that provide census transcripts would stick to transcribing what the census actually says, and not (mis)calculate the year of birth by subtracting the person's age from the year of the census, which gives a wrong answer three times out of four.
In 1891, the census says that she was 78, which, if accurate, suggests that she was born in 1812 or 1813. As these dates do not overlap with the ones deduced from the 1881 census, at least one of them has to be wrong.
She died in 1900 at the age of 91 in West Ham, Essex.
Again, if accurate, that implies that she was born in 1808 or 1809. That does at least overlap with her age as given in 1881, but not with her age as given in 1891.
All I have is the transcript not the actual record. If I can find that maybe it will have her and Catherine's parents names on it.
Sorry to disappoint you, but as Margaret died in England, her death certificate will not tell you the names of her parents. English (and Welsh, and Irish) death certificates do not record the names of the parents of the deceased.
You can order any English or Welsh death certificate from
https://www.gro.gov.uk so finding the actual record is not a difficult task.