Many females have habit of exposing their navel while they are dressed up in sarees. It is consider sensuous and they do it for seeking an attention of others. In Most of the cases, it is associated with personality. They do not find any thing wrong in this. Nevertheless, I agree with you, at certain age it is not desirable. Talk to her about this in very quiet way (I mean do not offend, advise or confront). You may think of taking help of your sister, if you have.