R.B.
Hi,
A quick thought that may or may not effect the thumb sucking. My kids are the same age span as yours will be, and my older one is a girl too. We threw her a "Big Sister Party" and had her help plan it and invited her friends that were already Big Sisters and Big Brothers. She felt really important and we helped her be a part of the process and honored her new role in life. A few of the gifts she received are still in her room 5 years later.
The thumb sucking may just be something that helps her feel in control and able to comfort herself.
Good Luck.