They are hitting a baby. Do you really need to think this through? If one of my kids hit a baby they would be gone that day. Family or not, the bottom line is that these two don't understand how to behave around infants. They are dangerous and need to be in a setting where there are no infants.
If my fourteen month old went home with one mark on him from an older child his parents would pull him that day. They expect me to insure his safety all day every day.