Getting a child to stop sucking fingers

My niece is a little over 2.5 years old. She never took to a pacifier, but has always wanted to suck on her pointer&middle; fingers (together) to soothe herself. Now that's she's almost 3 and still doing it we're trying to break that habit as it can cause for some serious dental issues and it's not good for her nails.

Tips and tricks to getting her to stop? We tried putting vinegar on her finger tips, but she's smart enough that she just went and washed her hands because they were "dirty".