Another snippet which may or not be useful. A Robert Dunning was buried on 18 Apr 1836 at Melcombe Regis. Age at death 34 making year of birth around 1802, so a possibility he was the husband of Mary Ann Woodsford. And a burial for a Mary Ann Dunning in MR on 2 Aug 1831, age 28 so birth ear abt 1803. So unfortunately no chance of finding her on the 1851 census and confirming birth place.
Hill being a more common name, it's a bit more difficult to trace the other (possible) sister. There is a burial for a Robert Hill (born about 1798) abode WR on 30 Nov 1826 in Wyke Regis (Location quite close and age a possibility). This would be two years after marriage. Can't find an obvious death for Sarah pre 1841, so perhaps she can be found on the censuses. Maybe she married again.
Wyke Regis baptisms has Eliza daughter of Robert & Sarah Hill on 2 Oct 1825. His occupation is sailor.
Added
I have found Sarah and Eliza on 1841 census in MR. Sarah is a monthly nurse and Eliza is a dressmaker. It looks like they are with the household of Christopher Goodwin and family. There is a 2 month old baby Goodwin, so perhaps Sarah is just there to look after baby. Sarah's age is given as 39 (born abt 1802). Might be reasonably accurate - looks like this enumerator didn't bother about rounding.
Can't find either Sarah or Eliza in 1851