Actually, the biggest ninny here is off-camera. That would be the human being who only put down one bowl of milk for these two cats.
We always put down two plates of food for our two cats. Instead of moving the plates around, they move themselves around. Eat from this plate for a minute, then go over and eat out of the other one. Meanwhile, your sister goes over and eats out of the plate you just left behind.
Repeat as often as needed to get human being to gnash his teeth.