Maybe I'm looking at it wrong, but on the census image :
Elizabeth age 64 birth +/- 1827 which you have
William son age 26 birth +/- 1865
Walter grandson age 15 birth +/- 1876
Ernest grandson age 14 birth +/- 1877
Which would have made Elizabeth around 50 when Ernest was born, and 49 when Walter was born, so quite probable to be their grandmother.