Ellen (or Hellen), but Mary Ellen on marriage record.
1841 is a problem. Helen was not with her family, but clearly the sister of Catherine. There was an Ellen (10) with John a labourer and Mary (35 & 30), at Liverpool Islington, but with siblings Daniel (8), John (6) and James (1).
In 1861, Catherine, sister-in-law, was living with Michael and Ellen Stokes (both 32) at Toxteth Park. Her father John Woodward (Wid, 56) was also present.