R.C.
This is common. I think you are on the right track by talking to your daughter, and when she gets defiant, I would just say "the sooner you go to the bathroom and get that done, the sooner you can come back to what you are doing." Be insistent, and be sure she understands that when she starts back to going by herself, you will not be telling her to go so much.
If your 2 year old is training now too, that could be some of it. There might be the sense with your older child that there is competition there or something. It is weird to me, but it happened with my two who were 21 months apart. Just a heads up. GOOD LUCK!! It will work out!