Your daughter sucks her fingers to help self-soothe and this is absolutely normal behaviour. Babies find sucking very comforting. A dummy, blankie, teddy, nappy, mom's hair, thumb and fingers are all 'transitional objects' that helo them to take in and make sense of the world - from the inside world of 'me' to the outside world of 'not me'. When children attach to these behavious we do not see any pathology in it - it's not a crutch or sign of insecurity. It's actually a sign that the child has found an independant means of coping with the world.
If your daughter's sucking is interfering with the development of her jaw, then you will have to limit it where possible, but you will need to check with a professional in this regard. She would then be taught to do it only in private and continually make her aware that she has to stop. Once she is older she will understand better and will have better impulse control.