Teeth brushing is a common area for power struggles to emerge between parent and child.
Young children often perceive brushing as intrusive and about as welcome as swallowing medicine. Parents often get even more emotional about the issue if the expense of dental work or orthodontics is burdensome. Parents either need to do it for him or finish up the job. It will give everyone a chance to calm down. Then try again in a month or so, but use a reward system to motivate her, not consequences.