There is the following 1861 Census entry:
RG9; Piece: 312; Folio: 94; Page: 49
13 Cross Street, Southwark, Surrey
George Nicholson Head Mar 34 Fishmonger Norfolk
Sarah " Wife 35 Essex
William " Son 12 Scholar Norfolk, Dereham
Sarah " Dau 12 Essex, Harwich
George " Son 6 Middlesex, Mile End
There is also the following marriage:
13 Dec 1868 St John The Evangelist, Waterloo, Lambeth
William George Nicholson, full age, bachelor, ?, Princes ? &
Sarah Bleach, age 20, spinster
Fathers: George Nicholson, fishmonger and Thomas Bleach, carpenter
Witnesses: ? Bleach and Sarah Bleach
The ages and places of birth for George and William in the 1861 Census agree with the information you have provided. With William and Sarah (Jr) both being aged 12 but born in different places, William is probably George's but not Sarah's son (and vice versa for Sarah (Jr)). Additionally, the 1851 Census shows William as being William G. Nicholson.
Charlotte Cooper Dunn was born on 22 Dec 1829 and baptised at East Dereham on 5 Jan 1830. Parents: Robert and Sarah Dunn (late Cooper). Robert was a blacksmith. Abode: Dereham.
George Nicholson was born on 18 Jun 1826 and baptised at East Dereham on 16 Jul 1826. Parents: James and Mary Ann Nicholson (late Hopson). James was a butcher. Abode: Dereham.
If these are the correct George and William, has Charlotte died before 1861 or have George and Charlotte separated?
Nigel