You need to provide more information in order to get a good response that suits to your situation. How old are you? How old is your chachi? Does she touch you when you are alone? Does she touch you even when you are with family? Most importantly, do you want to be touched - do you like the experience? or are you the victim of abuse?
If you are not being abused - i.e., if you like being touched, then it is a different situation. Growing up, we are taught about certain values and sentiments about our family - you respect your brother's wife, or your sister's husband, you respect your uncles adn aunts. But times are changing and so are the values. if you feel that it is okay to have fun with your chachi, no one can stop you, provided your chachi likes it as well. So try to figure out if she likes you. Does she touch you all the time or only when you are alone? Where does she touch you? Many times, we would like to think that we are being seduced or provoked, but we may be wrong. More details might help in trying to figure out if its a case of misguided love as an elderly family person or if its down right seduction.
However, if your chachi is taking advantage of your situation, it is really sad to experience such an abuse. You need to tell your parents, preferably your mother and let her know whats happening. Although in India it is not likely to lead to a confrontation, your mother might intervene every time your chachi comes to 'see' you when you are alone in your room. After a while, your chachi might understand that she is being interrupted and might stop her from approaching you.