Lancashire Death indexes for the years: 1839 ALSOP Sarah A 36 Eastern Bolton EB/3/181
ALSOP, SARAH ANN 36 GRO Reference: 1839 S Quarter in BOLTON UNION Volume 21 Page 57
This is the only Alsop death and no births in Bolton (1837 to 1841), so unlikely Sarah Ann died in childbirth
General Notes
Sarah Ann is too old to appear on 1841+ census records, as by then she would have been aged 38, married and left home.
1803 Baptism record confirms parent as John and Mary, so presumed married by this time. This Baptist Church was only founded in 1803, so parents John and Mary may have had other older children baptised elsewhere (so not found).
Searching for "Buckley" marriages on the same IGI index as the marriage for brother Corneleus (M007153) reveals 18 marriages and one is "SARAH ANNE", so this is maybe her.
Also, Sarah Buckley may have emigrated to Australia, married Henry Wild, and died in New South Wales in 1851 (a suggestion from Matthew vai GenesReunited).
She may have been a twin to Elizabeth Buckley, who emigrated to Utah, USA in 1853 or a step-sister if John had married twice.