Something I don't see in the original post...approximately how old are you and your husband, and how long have you been married? 

It looks as if your sister in law is not adversarial....that is the number one thing that can make this work smoothly.  If she is on the same page as your husband, it will go well.  

Just make sure you understand the difference between making money and spending money.  Just because one doesn't put a lot of dollars in the checking account to spend foolishly does not mean that one is not making money...in fact, the person who puts the money back into profitable enterprises will come out ahead in the long run.....far, far ahead.