My 20 mth old sucks her thumb too, and I know how much comfort she gets from it, so I can understand why you want to go gently with your daughter. I don't encourage or discourage my girl from sucking her thumb because I sucked mine for many years as a child and my teeth are straight; I've never had braces.
I feel sorry for your sister's teeth trouble, but I don't think it's as common as some people believe. I find that once I talk to a few people and admit that I sucked my thumb as a child but never had teeth problems, a lot of people admit to doing it too (and often, had no teeth trouble). I think there's such a stigma against it that even adults don't like to admit having sucked their thumb as a child.
As for getting your daughter to stop; I remember deciding it wasn't cool and stopping that night! SO, I figure when they're ready, they'll stop themselves. Anyway, thats' just my opinion!