Harassing by neighborhood

One of my neighbors is my own aunty (my father’s brother’s wife) and her son. We are sharing the backyard as the property is not yet separated by law. Of course after separation I will be the owner of the 50% of the total land (including the backyard). Many years back, my aunty constructed an “Underground Waste Water Tank” to accumulate the waste dirty water of bathroom etc. in the middle of the backyard. I did not oppose at that time as of mutual understanding. But now the problem is that from last 12-15 months it(the underground tank) is badly overflowing and all the water covered the whole backyard at all the time. This is a very annoying situation for me as its looks very dirty and smells very very bad all the time. As my bedroom is near the backyard, I, my wife and my 5 years child even cannot sleep properly specially during the rainy seasons. I talked about the issue with her but she did not take any proper action. She is a very rough lady and somewhat politically strong. So I am in very frustration. Can the Law help me? If yes, what is the process?