D.S.
You have probably figured out that it is not about the potty, it is about control. If you give her control over it, and don't intervene (change her, clean her up, etc.) she will likely decide it is not worth it. It takes a lot of tongue bitting, self control from you, but it shouldn't take more than a day or two for her to just get tired of being wet or dirty. Put her in regular underwear and stay with her in a part of the house you can clean. Maybe get two potty chairs and ask her to help you train the younger one. She is likely jealous of the attention the younger one gets while being diapered or whatever. If you gave her control over something else (what clothes she wears or what she eats for a while) she may be more willing to yield on this issue.
Whatever you do, do not reward her for non-compliance. If she can get a reaction from you, no matter how negative it is, she wins and the game goes on. If you make it her issue and hers alone, she will soon decided it is in her best interest to get this thing done.