A new baby often makes older siblings regress, much more than we will ever know
The new baby has all the attention as far as she sees things and I would lay money on this being her issue
For the nursery staff to miss wet and/or dirty knickers isn't to surprising TBH and easily done, however, I would encourage your DD to tell them if she does this - and to not say anything if she forgets and dont say anything about the nursery staff in front of her either. Speak to them out of her earshot
It is incredibly hard to keep patient, believe me I know, but one small sign of frustration, annoyance etc and it will be back to square one every time
The fact she is interested in her brother's nappy doesn't mean she 'gets it' but is more likely to be 'if he can so the heck can I!'
She wont be doing it for ever
GL