The 1911 census tells us that Albert and Jane had three infant deaths, and Lucy is the only infant death for a Stocks found in Keighley while they were living there. Therefore, Lucy was probably a daughter of Jane and Albert.
She was born several months after their marriage and died at birth or soon after. She may have been named after Jane's future sister-in-law Lucy Hill, although Lucy didnt marry brother William for another three years.
There was another Lucy Stocks who died as an infant in Q3-1880 in Birkenhead, but Albert and Jane were still in Bingley in 1881.