Depression. She doesnt know where he is?

My wife is in depression mood. We have a baby child 1 1/2 year. She was taking depression tablets for the past 3 years. After delivery she was not taking those tablets. My wife is thinking about the person when she was loved in her college days, but that person rejected. Now she doesnt know where he is? She wants to leave me and my baby now and wants divorce. She is tellingthat wants to marry my elder brothers son, because that person looks like she was loved in her college days. She always have a doubt on me and my sister-in-law. She doesn't do any work in my house and even she doesn't caring my baby also. Mental harassment for me. Please give a better treatment for my wife beacause i dont want to leave her. I want to stay with her only.