You’re right, David --- we haven’t established beyond doubt that the Wm Crebbin buried Braddan 1840 was the one who was the husband of Sarah Fell.
However ----- I’ve used the bits of information we do have and I’ve made a jig-saw puzzle of them. Here are the pieces:
1) the MI:
MI, Braddan Old Yard North: Here repose the remains of Mary Ann Crebbin, daughter of Alexander Crebbin of Douglas, who departed this life the 6th of Oct 1837, aged 20 years. Also Isabella Crebbin, daughter of W. Crebbin, who died the 6th of Oct 1837,aged 6 years.
2) Baptism of Mary Ann Crebbin: 29 Dec 1816 Malew to Alexr Crebbin and Ann Brew.
3) Baptism of William Crebbin 25 Sept 1811 to Alexr Crebbin and Ann Brew.
4) Baptism of Isabella Ann Crebbin 27 June 1832 to William Crebbin and Sarah Fayle St Mathews Douglas.
5) Burial of Wm Crebbin 24 Apr 1840 age 29 years Braddan.
6) Burial of Mary Ann Crebbin 6 Oct 1837 age 20 yrs Braddan.
7) Burial of Isabella Ann Crebbin 7 Oct 1837 age 6 Braddan --- but transcribed as CUBBIN (it would have been very unusual to see the name Cubbon written as Cubbin on the IOM in 1837)
I’ll attach a screenshot of the burial register page which shows both Mary Ann’s and Isabella Ann’s entries.
It all seems to fit together pretty well if you make allowance for the fact that Crebbin is one of those names which needs to be written carefully in lower case letters in order that it won’t be mistranscribed.
I have a mental image of 2 very ill girls age 20 and 6 who die within hours of each other of something contagious, who are buried together within hours of the deaths, and who are named together on the same headstone ---aunt/niece.