How To Let My Relative Move Out From My Home

The relationship between me and wife is not good. We have many argues and quarrels because we always have different ideas. We have two elementary school kids and own a small single family house with three small bedrooms. The relationship has been getting worse between of us since my wife's sister who is single moved in a few months ago. She doesn't work and stay in our home most of time everyday, so she got most of my private information and liked to tell other people about my privacy that I don't like to let other people know. In addition, she often helped her sister argue or quarrel with me. I really don't like her to live with us any more. I told her and my wife that I hoped she move out soon, however, she refused because my wife insisted to let her continue to live with us. She lives in our basement. I feel really inconvenient and frustrated for her living with us. How should I do? How should I let my sister in law move out? Thank you in advance!