Also what Sylvia did say is perhaps he is buried near his brother james ?? Not that I can find where he is buried either??
I don't believe he had a brother James.
Not sure what sites you are using. But some links here to Familysearch. It is free, but you do have to register.
1939 -Hannah & Harold Jackson can be found in 1939
Harold Jackson was a twin
these births -
Harold Jackson dec qtr 1913 Burnley 8d 436
Lilly Jackson dec qtr 1913 Burnley 8d 436. mmn RICHARDSON
(so Harold was a year out in 1939) But Lily got it right
William JACKSON and Clara RICHARDSON were married Burnley Mar qtr 1896
A son - Latham Jackson S. Qtr 1897, Burnley Union. Vol 08E page 163
1911 -
https://www.familysearch.org/ark:/61903/1:1:X4M2-8HY(the twins must have been a big surprise!)
Anyway - Latham has a WW1 record that has survived
he married Dorothy HILLARY in 1919. I don’t see that they had any children
and died 23 August 1976 there is a probate for him. Cannot see his burial either. But perhaps it is he Harold was interred near.
William, Clara and Latham look as though they tried living in Canada. They are on a ship to Canada July 1906 going to Winnipeg Also an Elizabeth Jackson age 35, single - it might say “to stop with brother-in-law” not sure
and an Elizabeth A. Wilson age 18
https://www.familysearch.org/ark:/61903/1:1:2Q3D-W5R.
Harold’s Twin Lily died in Halifax on 2 July 1964 (Lily HAIGH wife of Eric HAIGH) They were married in 1938
Possibly a son in 1940 - Halifax and a daughter 1941 - would need certs to confirm - so they would be your m-in-law’s cousins.
Lily shows on Deceased on line as a cremation
https://www.deceasedonline.com/servlet/GSDOSearch?DetsView=Summary&src=ext&fileid=14024538https://www.freebmd.org.uk/cgi/search.pl