One option would be to take away the pacifiers from your younger one, so your older child doesn't have access.
My older son never used one, but my younger son did. When he was four, the "Nuk Fairy" came and took them away, and left behind a gift. He talked about his nuks off and on for a year, but managed.
When he was three, we restricted the use of the nuks to his bed -- they could only be used if he was in his bed. So when he was feeling stressed, he would self-soothe by going to his room and using his nuk for a while. When he was four and the nuks were gone, we noticed that he came up with a new self-soothing technique. He sat on the bottom step of the stairs that came into the kitchen, right next to the shelf with the library books, and he would read a book.
Help your daughter find a way to calm herself, and she will probably miss the nuk less.