How is your relationship with your niece now?

My general thinking would be to try to repair things with your niece if possible and forget about the other people. It is indeed exceedingly annoying when people self-righteously and hypocritically criticize. But how will it help things with your niece if you engage in back-and-forth with K? If both you and K engage, the back-and-forth will go on indefinitely.

You could consider being the bigger person, which in my opinion often means owning up, fixing, and moving forward in a better way.