It looks as though there was a possible sibling
Births Mar 1873
Curwin John Henry MMN Johnston
Whitehaven 10b 629
Death June qtr 1874
Curwen John Henry age 1
Whitehaven 10b 406
Henrietta's birth certificate may not name a father. The entry for her mother would read as Mary A Curwen and give her previous surname ThompsonJohnston that could be where the information on the index came from.
ADDED
Mary Ann is stating she is widowed in 1891, I cannot see her in 1881. I can't see a death for Thomas either.