AITA for telling my husband I dont want his younger sister living with us?

Sorry for format ,on mobile and writing quickly. Backstory: My husband (28)and I(29) have been married for 3 years and now have our own home but lived in our IL basement for the first 6 months of our marriage. It was a last resort situation until we could purchase our home. Entire 6 months we paid rent, bought our own separate groceries , and maintained all our belongings downstairs. My personal belongings started missing from downstairs including a Fitbit, makeup, and clothes and I started getting suspicious of my younger SIL. Turns out suspicions are right, and my belongings are "discovered" in her room. I had a talk with her parents and explained how upset I was blah blah blah. They spoke to her ,she never apologized and life went on per usual.Fast forward to 6 months ago: SIL has not learned her lesson, gets in trouble for shoplifting and IL have to pay fines, etc.Now: IL have marital problems and apparently life is a living hell for younger SIL. She complains to my husband almost daily about her parents antics (they truly are acting like children to spite eachother). She hints at wanting to leave the house and husband feels stressed and sad his sis is having to deal. He hints at getting her out of that situation by having her live with us.I told my husband that Im truly sorry, but that I don't feel comfortable having his sister living with us because of her habits. Shes studying from home because of covid and she would have free reign over our house until we could get back from work. Its his parents responsibility to provide a good household for their kids and I dont feel the current situation warrants her stay at our house. Taking care of a teenager (that isnt ours) is a whole different responsibility I feel bad and want to be supportive but I feel like we're just opening a can of worms. For clarity, if she was in dire need of a home or (God forbid) something happened to my IL I would bite the bullet and have her stay, but not for marital problems via /r/AmItheAsshole https://ift.tt/2T8pJrX