First of all, don't make it about someone else. Don't say something like, "You are not going to use diapers anymore because diapers are for babies and you're going to be a big sister." Keep it simple and focused, "You are going to wear underwear now, and you can pee pee and poo in the potty but not in your pants."
Second, take her out of the diaper (except perhaps at night). She will not care about using the potty as long as she has a diaper to pee in and someone to change it. It will mean busting out the Resolve a few times, but stick her right in underwear and set her on the toilet often. Yes, she will probably soak herself a few times. She needs to feel the wetness and and pee running down her leg to realize she doesn't like it.
When she pees on herself, don't scold her, but have her feel it with her hand so she understands she's wet. Have her remove her underwear herself as best she can and put on a new pair as best as she can. She needs to be in control of the process and realize that her bodily functions are no longer mama's job. Then calmly tell her to clean up the floor (and go behind her later to really clean it). Have her feel her new dry panties.
For my daughter, as long as she was in a diaper she would pee in it. The first time I put her in the CLOTH pull up pants, she stayed dry and peed in the toilet. She is 2 years and 2 months old. It seems funny, but I would praise her for just farting on the toilet because I knew that it meant she was making an attempt to push. Also, at that age they are usually capable but don't want to sit long enough. If necessary, let her sit on the toilet with a book. Encourage her to stay on but let her off if she starts getting upset.
For the average child her age, a few times is all it takes. Night training might be different depending on what kind of sleeper she is, but for my son (now 5) the same process worked for night. I day trained him but assumed he needed a night diaper. As long as he had it, he would pee in it. Finally I just put him in underwear and then the hard part, for a little while I would make myself or husband get him up in the night to go. Eventually we quit and he had no problems getting himself up to go, although he needed help to pull up his underwear. He was in a toddler bed so he could easily get in and out.