I suspected she hadn't been named Eleanor when I saw it was her sister's second name. If you say "Eleanor" then "Helena" they sound almost the same, so it would depend who was filling in the particular record, as to how accurate it was. The 1891 census was being filled in by an enumerator who would be depending on what he heard.
Who registered her daughter's birth?

Are you thinking that Sarah might have been Marjorie's mother? (But she was the wrong age for Eleanor in the 1891 census record you found.) Can you find Sarah in 1891 to exclude her?

I found Marjorie Crowe, 6, in Frodsham in the 1911 census, as adopted daughter b.Wirral Woodchurch Cheshire, but I'm assuming you already have this record?

I couldn't find Eleanor/Helena in the 1911 census under the married name given on the link Jean posted, or under her maiden name.

Sue