Hi Sarah
Really please don't worry, she will do it when she is ready and hard though it is, ignore the grandparents and any other people trying to pressure you! I have worked with children for 20 years now (OMG thats scary when on paper!) and as I say have three girls and two boys, not sure about number six yet! My youngest is 2 and a half, although she tells me when she has done a `big stinky poo' is not remotely interested in the toilet or potty. Regarding the younger granddaughter, I guarantee she will not stay dry, a friend of mine was so pleased when her son was dry at 18 months, and has not stopped moaning since he turned two that he is constantly wetting himself again.
We are all different and do things at different times, just give her lots of praise and hugs when she manages to go on the toilet and try not to be negative when she has an accident, regarding treats and rewards, they work for some children, but it always makes them think they are in control and they soon realise they have the power!!
Big hugs and do not worry, she is perfectly normal and you are doing a great job as a mum, in fact we all are, it is one of the most rewarding but hardest jobs EVER !!
Mandi xxx