What are the consequences of non adoption of stepdaughter

Hello respected lawyers, I need some legal help to understand the consequences of non-adoption of my stepdaughter. She is 5 year old child of my second wife. We three live together. My wife had an abusive first marriage (Hindu marriage law). She has custody of her daughter and her ex-husband has monthly visitation. However, her ex-husband doesn't turns up regularly to visit his daughter. However, my wife clearly knows that her ex-husband will never give his consent to allow adoption to me. I'm not sure how to approach this. If it continues like this without legal adoption what are the pros and cons to me in future? Can anyone help? Thank you