My SZ sister is looking for the perfume she bought using the money she stole from my mom and thinks she still has it in the house

My mom told her she already returned it and as of right now she’s still looking for it and keeps demanding my mom to know where it is even though she keeps telling her over and over again. What should my mom do? Should she give it back? Buy her a new one? Or what? Please, advice is needed ASAP!

This book is very helpful in helping people with SZ: I Am Not Sick, I Don’t Need Help! How to Help Someone Accept Treatment.
If she’s not taking meds and seeing a psychiatrist, that would be the first thing to get her to do.