M.R.
My sister did this when she was little .. same fingers too! Not only did she get sick all the time but she had braces for many more years than she should've because it pushed her front teeth forward as they grew. They do sell things that you can paint her nails with that tastes absolutely horrid and although it sounds terrible it worked. Within a week she was done sucking on her fingers. You mentioned self soothing also and perhaps you could get her a "special" stuffed animal and explain to her that it will take her tears away when she holds on to it but that her new special friend can't do that if she has her fingers in her mouth. Also they have balls with "feelers" on them that can give a great soothing effect just by touch. My son has one of those and I find myself touching it over and over again as well! I would definitely get the nail polish though, it really does work!!