Hello Deep, I think you should sit and sweetly talk to her about this habit and be a party to heading towards the solution. Make her think that you both will work towards leaving this habit. And always be appreciative of her effort no matter how small it is. Say, if she doesn't suck her thumb for a few hours acknowledge it and praise her. Praise her in front of other family members and what good job she is doing to leave the bad habit. http://www.webmd.com/parenting/features/9-ways-to-wean-a-child-off-thumb-sucking Go through this link, hope it will help you.
Hi, I think distract her whenever she does that. Involve her in some activity or the other. Tell her how germs are going inside her and they would grow in her tummy. Sometimes, such gory details about the germs repulse kids and they correct their habit. I used tos uck thumb myself, and left it immediately when my mother scared me saying that your teeth will come out like that of an elephant if I continue to do so. Also, you can treat her if she doesn't suck thumb for a specified time. Eventually you can increase the time of her not sucking the thumb.
thanks Mam we do all these things but all-waste moreover we used thumb paint, for sometimes she left her habit.... but as time passed she also used to that paint but she didn't left her habit :)
Deep Sunny Ahluwalia