Abandoning house & going with a person of her choice

My Sister's daughter aged 20 years who is in her final BCom decided to move out of the house & go a with boy of her choice who is 24 years & not completed his 10th & does not have a job. My sisters husband has abandoned them around 4 years & he is staying alone while my sister was taking care of her daughter. The boy's parents are dead & he is staying with his brother. Her decision of going with him was firm & she has given it in writing on a paper in her own writing saying that she is going on at her will & will not claim anything from us, even the boy has given a letter saying that he is taking her, both the letters has been signed by some representatives who had come on boy's behalf. In our interest & protecting ourselves for future consequences do we need to inform this to police so that things gets registered & our liability seizes. We belong to Brahmin & the boy is Gowda.