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