This is perhaps your family but then Ann wouldn't be the right age to marry in 1851 
1871 RG10; Piece: 3936; Folio: 10; Page: 13
70 St George's Rd Bolton
William Hewitt 34 yrs Wholesale Tea Dealer employing 3 men and 3 boys b Lancas Bedford Leigh
Ann Hewitt 33 yrs b N Wales Montgomery
Mary E Hewitt 10 yrs
Clara Hewitt 7 yrs
William Hewitt 5 yrs
Thomas H Hewitt 1 yrs
Ann Williams 20 yrs general servant b Flintshire Whitford
Elizabeth Roberts 17 yrs Nurse b Denbigh Abergele
Does any of this identify the family to you?
heywood
Ann Williams who is the servant in this census is more likely to be related to Catherine and Thomas Williams -it looks like Thomas gives his p.o.b. as Whitford Flints
I think you really are mixed up with this - and you need to be as accurate as possible as looking for Williams in Wales can be very complicated.
Please could you tell us what information you have
Catherine Barbara Williams was the daughter of Thomas
could you tell us which census you have Thomas in and his d.o.b. and p.o.b.
Could you also tell us where Ann Williams fits in - at first you were stating Ann was the sister of Catherine - now you are saying Ann was Catherine's neice -daughter of her brother Thomas -but Catherine's father was Thomas?

Suz