Lola did this around 19 months or so. I think she just wanted to be like her big sister!
One thing that helped me was I had a 'if it works it works' attitude and haven't ever stressed about it. Even with accidents I have the 'oh well she's only just two" kind of thoughts rather than the "OMG she should know better" kind. Its really stress free!
Good on Niamh .... no more fluff though