You say “all of Caroline’s sisters died in England” but she only appears to have had 1 sister
1841 census for her parents John & Ann - 2 sons - John 6 & William 5 mths
1851 - Caroline 5 & Ellen 8mths
1861 - Ellen aged 10
Mother Ann’s maiden names was Oakley
https://www.familysearch.org/ark:/61903/1:1:N66Q-G2N
BULBECK, ANN mmn OAKLEY  
GRO Reference: 1837  D Quarter in CATHERINGTON  Volume 07  Page 47
Death:
BULBECK, ANN  0  
GRO Reference: 1838  M Quarter in CATHERINGTON  Volume 07  Page 51
BULBECK, ELLEN mmn OAKLEY  
GRO Reference: 1839  M Quarter in CATHERINGTON  Volume 07  Page 52
Death:
BULBECK, ELLEN  0  
GRO Reference: 1839  J Quarter in CATHERINGTON  Volume 07  Page 44
BULBECK, WILLIAM mmn OAKLEY  
GRO Reference: 1840  D Quarter in CATHERINGTON  Volume 07  Page 55
BULBECK, CAROLINE mmn OAKLEY  
GRO Reference: 1846  J Quarter in CATHERINGTON  Volume 07  Page 69
BULBECK, ELLEN mmn OAKLEY  
GRO Reference: 1850  S Quarter in CATHERINGTON  Volume 07  Page 67
BULBECK, HENRY mmn OAKLEY  
GRO Reference: 1853  M Quarter in CATHERINGTON  Volume 02C  Page 116
Jamjar