Hi Karen
According to Familysearch IGI Janet MCEWAN born 10th August 1866 at Calton Reg Dist , Glasgow. Father Andrew MCEWAN, mother Margaret MCCULLOCH. The IGI also has siblings Alexander MCEWAN born 12th Nov 1868 Calton Reg Dist
Thomas MCEWAN born 1st July 1864 Old Monkland Reg Dist, Lanarkshire.
In 1871 census the family were in Scarborough, England but were back in Scotland in 1881.
You say Janet was present at her mothers death, does the certificate show both Janet's married name, Mcfadyen, and her address?
Do you have a copy of Janet's death certificate? What does it say for her parents? Also, is the address the same as Janet gave (if she did) on her mothers death certificate?
There should certainly be enough documentary evidence to show that the marriage certificate is incorrect.
The other thing that would be worth checking is the Parish Register as duckweed suggested in an earlier post, this was probably signed by the bride and groom when the marriage took place. The certificate would name the ministers church then its a matter of finding where the OPR's are.
According to the 1901 census, Henry was born in Co Tyrone, Ireland. The IGI does have 3 births in Tyrone for parents Henry MCFADDEN and Eliza Jane IRVINE
Aron born 6th July 1867
Thomas born 17th May 1865 (mother Eliza IRVINE)
Mary Anne born 17th Jan 1871
but no mention of Henry who would be older.
Thomas & Mary Anne do appear to match Henry's siblings although Mary Anne's age on Ancestry in the 1881 census is wrong and could be a transcription error. To verify who actually was Henry's mother you would need to find his birth or baptism. Its possible that Eliza Jane was his step-mother.
Andy