Hi D.,
I sucked my thumb until I was 4 1/2. My teeth are perfectly straight...my husband told me that he wonders if my teeth would have been indented and sucking my thumb actually perfected them.:) Anyway, my parents actually bribed me into stopping. My Dad told me that he would buy me anything I wanted in the "Sears catalog" (this was in the 70's). It worked for me. He bought me a doll w/ playpen/stroller the works. I remember pulling the cover up to hide at night and suck my thumb and my Mom would say, "if you don't stop, we're going to have to send everything back." I did it on my own but, it took a little time (a few weeks). My Mom would reassure me every night that I could do it. My own daughter had a pacifier and I tried everything to get her to stop. She started reading when she was 3 but, had a pacifier. I caught a lot of flack. I tried everything...I even told her she could only have it in bed and she would stay in her bed all day. I had to sleep with her for about a week when I finally just took it away. I remember it was a heartwrenching time. Our dentist told us that as long as we got her to stop before her adult teeth started coming in, it would be ok. She ended up with braces anyway, just because she has her father's teeth. I have heard of the appliance that they put in and I'm sure it works. I think your niece will stop; it just takes time as with most habits. I think that your sister should just keep encouraging her that she can do it. Give her a chance. It will be a good accomplishment for her to look back on when she is older. My heart goes out to this child and mother...it's a hard habit to break. My daughter was such a great baby and I know that a lot of it was because she was always pacified!