How Do I Make Her Stop?

It may be hard to get her to stop completely but what my did with my sister was told her if she wanted to suck her fingers she had to go to bed to do it. My sister ended up sucking her fingers until she was 11, but only to go to sleep and she did end up needed a retainer as a teenager. She is now going through the same thing with her 4 and 7 year olds. Its really hard to get kids to break the habit.