I lucked out big time and stepmother in law stayed out of my kitchen. She came in and talked to me some, which was fine, but she didn't get in my way and didn't get waspy when I wouldn't put her to work.
I lucked out again on dish duty after dinner. My son wanted her to watch a movie with him and I was able to get her to go watch the movie by saying "Oh please go ahead, if you guys don't hurry up and watch that thing it's going to be midnight before we all get to bed!" It worked like a charm!