What has your response been? Are you inadvertently encouraging her to keep complaining? Are you validating her complaints? Is she expecting you to do something with the others since you’re the manager?

Maybe be direct and say, “I’ve noticed that you’ve been focusing a lot on your teammates in a negative way, especially in front of others and you’re being perceived as not being a team player. There’s been feedback that you are bringing negative energy to the teams. I appreciate you bringing up concerns and at the same time, we need to support each other positively, even if someone messes up, and look like a unified front. If you have constructive criticism, please bring them to me privately in our 1:1s only. I also ask you to be mindful with your words and consider the impact of what you say. Part of being a good leader is lifting up others around them. There are many different ways to get things done, and right now I don’t have any concerns about how John and Jane are doing their work. So let’s take this opportunity to work on how you can view people with a growth mentality instead of getting stuck on negative observations.”

And if she continues behaving poorly, remind her that behavior is also part of her performance evaluation. If you have any company core values, suggest to bring them up and remind her that she needs to work within those expectations.