If its once in a rare while in an emergency, I'd let it go but if it is becoming regular, you, the nanny and the other family should sit down and talk about some share guidelines etc. to make sure everyone is on the same page. Also does she do this on days you need help because you asked or does she also do it on your regular days. Again, if it's once in a while to come to your rescue, I'd let it go but if it's regular or not planned by you, I might say something. If your arrangement is slowly morphing into a nanny share, you should treat it as that.