This is interesting I found a Louisa Marwood on FamilySearch.org boarding at the RC Convent Of Notre Dame in Blackburn the 1881 Census. She’s burn in Bury rather than Blackburn where the Marwood family would have been living at the time. It also has her down as being born in 1873 rather than 1862 which is the date my cousin Hamish has listed for Louisa.
Now if this Louisa is the sister of my great grandfather Frederick T. Marwood and she was born in 1873 rather than 1862, then it would make it more likely that she was his half sister and the daughter of his second wife (most possibly the Mary that died in 1884 Jul 25). I think this might be likely though.
Name: Louisa Marwood
Birth Year: 1873
Birthplace: Bury, Lancashire, England
Relationship to Head of Household: Boarder
Marital Condition: Unknown
Profession / Occupation: Scholar
Address: Convent Of Notre Dame Educational R C
Census Place: Blackburn, Lancashire, England