I wouldn't worry too much about cleaning, but definitely put away the clutter so she can start with a "clean slate".
If you have any "rules" regarding cooking or food limitations for either the sitter or the kids, jot that down and point it out to her. The kids are going to have a great time with her, but remind them of the "house rules" before she arrives and you may want to jot a few down for her too!
We don't get help from my husband's family so we have a date night sitter (college student) and we LOVE her! My son will ask when "Miss Megan" is coming over to play.
The only other thing that you may want to do (we do it) is call the neighbors and let them know that you will be out for the evening and the sitter will be at the house- just in case!