Hi Dalesman
A good place to start checking for possible siblings to Janet in the period you are looking at is both census entries for the family and also on IGI at
www.familysearch.org/Eng/Search/frameset_search.asp?PAGE=igi/search_IGI.asp&clear_form=true (which is great for Scottish births and marriages up to around 1874/5 when the Mormons stopped loading on Scottish data.
To check for siblings, on the search screen, simply put in full parents' names on the r/h side (James Chapman and Janet Henderson) and the British Isles/Scotland and you should see 5 entries for children between 1865-72, including Janet's entry in 1868.
Parents' marriage shows on 23 JUN 1865 in Kirkliston, West Lothian.
Images for these births and marriage are available to view on line at
www.scotlandspeople.gov.uk The marrige cert for parents will contain details on their parents to let you work back to the next generation.
Possible entry for the Chapman family in 1871:
James Chapman 26, quarryman b. Abercorn, Linlithgow
Janet Chapman 26 b. Dalmeny, Linlithgow
George Chapman 5 b. Kirklinton, Linlithgow
William H Chapman 4 b. Kirklinton, Linlithgow
Janet Chapman 2 b. Abercorn, Linlithgow
Margaret A Chapman 11 months b. Abercorn, Linlithgow
William Smith 31, lodger
Address: Woodend
Monica