Any software to find out employee stealing?

I am helping in my sister's restaurant. Recently, I found one of the employees steals the tips. I haven't exposed the staff because she always works hard and she has worked in the restaurant for quite a long time. I don't know how long she has been doing this. I talked to my sister but both of us are now in a dilemma. We think we should install more cameras to get the evidence of her stealing, then go talk to her. But we still want to know if there is a better way to solve this. Please advise, thanks.