Speak to your husband and tell him how important a help is to your household right now, and if she leaves, there will come many issues in your job and stress will automatically be there in your both life as a whole.
Tell him to talk to his mother and make her understand not to nag the help and you will give her instructions whatsoever.
You can also speak to your maid and take her in confidence and tell her not to mind your MILs behaviour as old people have peculiar problems of being dissatisfied with everything especially with their DILs.
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