What should I do about a loved Sister- book keeper, who has stolen from the company?

She has personal financial problems (self inflicted) and now is sneaking money out of the family business. I used to trust her with the finances but I have now given her two chances to stop but is now at it again. I love my sister and family but she is already overpaid for the work she does in this small business.I have been signing the cheques for the past two years but now she has even forged my name on some cheques made out to her and her husband (a key employee). He has been devastated by their surmounting debt. He only found out when they were drowning in it. I have to protect the company money or we will all be in trouble. We are a close family and I love both my sister and husband.