My brother sucked his thumb for years. My parents were beside themselves, and the doctors offered little help. Well he is now 36, happily married father of 2! I asked him a few years ago how he stopped and he didnt even know. He has always had the most beautiful, straight teeth, no braces needed, so much for the theory thumb sucking hurts teeth. My point, my parents worried for years and tried everything under the sun, for nothing. Based on their experience, if any of my children were thumb suckers I would not worry about it.
I'm glad your brothers teeth were never effected....but my dd's teeth have been very much affected and she is 5 1/2. One of her front top teeth is significantly lower than the other (it drives me crazy I'm ready for them to come out) - I just wanted to point this out because I had a little bit of the "she'll stop when she's ready mentality" and now I wish I had been more proactive in helping her stop.