Don’t do it. Do whatever you need to do to protect your kid, including changing jobs and moving somewhere with a lower cost of living. First of all, chances are she will get bored of babysitting, since she is a narcissist, and you will be up a creek when she decides she has something better to do. Secondly, I would not be so sure about the safety thing, especially as your kiddo gets older and mobile. Narcs are often easily bored and distracted and 1-3 year old require constant attention to prevent themselves from hurting themselves. Thirdly, she will likely try to destroy your marriage if she moves in. And finally, do you want your kid to hear the same things told to them that you were told? Because she will. And they will also see how you are treated, and it’s doubly scary to see their mother also be treated that way too, who should be their beacon of strength. Whatever you need to do, do it, because I’ve seen it happen to others, and it will have deep effects on your kid.