When I first got engaged, my father told me to be careful because you are not just marrying your beau, you're marry his family too. They are going to be in your life too. I have been lucky that my future mother in law only helps where she is wanted. She is paying for the rehearsal dinner so she has gotten to plan that within the theme of our wedding.

I would recommending giving her a job, like you picking out the favors and having her help put them together. Therefore she will feel involved without feeling like taking control. Hope that helps!