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Its not a bad thing to have your mother in law around to help out. Since she isnt getting it right in the area of cooking, talk to your wife about it.
She should use wisdom in telling her mother how the food should be cooked. Since its her mother i dont think they should be an issue.
People cook same things differently. She would just tell her mother how she cooks for you.
And when you are less busy, you can decide to do the cooking yourself and suggest that you are just helping around.