I'm sure as a parent, it's very disturbing for you to suspect that your child is stealing from you or her brother. I understand your concern and am glad to hear that you want to be sure that she's doing it before you accuse her. The only way to be 100% sure that she is stealing is to see it with your own eyes. If you have strong evidence that your daughter is stealing, have a calm conversation with her about why it's wrong to steal and the consequences she will face because of it. Explain that if she continues to steal people will have a hard time trusting her and will be less likely to want to be associated with her. It will be more sincere if you see her with your own eyes as opposed using a camera or dye packet.