Hi
Sorry, I can't find her in the 1871 census but have possibly found the family in 1881:
Robert Cubbin  Head age 41 b Devon
Martha Cubbin  Wife age 41 b Portsmouth
Alice Apsey   Step daughter age 13 b Portsmouth
The address is 69 Somers Road Portsea.  The wife's name is not Sarah though so maybe Sarah was a middle name??
You may have this already, sorry if you have and also sorry it is not 1871.
Hope this helps.
Regards
Victoria