In this case she has the kids every other weekend. If she can't pay attention to the kids enough to realize when they can or could not hear them and/or can't choose to be quiet enough that the kids would notice even if they happen to be awake, that shows a blatent disregard to the kid's best interest and could be a symptom of other selfish behaviors that are destructive along with possible self control issues that could effect the safety and well being of the kids.
But, we really don't have all the details here.