Just like adults that have to pay the price and deal with the circumstances of doing wrong, whether an adult gets caught or turns themselves in on their own, there are consequences. By telling your child that if she tells the truth she won't get in trouble is not teaching her what reality of life is once she is an adult. She will be shocked when she gets out on her own.
Because all children are different you have to find what works. If her punishments are harsh if she gets caught and her punishments are lighter if she comes and tells the truth this may work for her. There has to be clear unquestionable boundaries but either way she knows by doing wrong, which everyone makes mistakes or makes wrong decisions she will have to be punished. This may seem to her that she can't win but making the "harsh to light" punishments very different she will see that telling the truth may still get her into trouble but better to lose a day or two of TV than getting a spanking and being grounded for a week with no privileges. Obviously as she grows older the punishments will have to change and the severity.
Make sure she is saying she is sorry to anyone, including you as parents when she has wronged someone but also making sure she understands why it is wrong. Most importantly though, make sure when you do punish her that you explain why she is being punished, ask her questions to make her answer you as to why what she did was wrong (this helps children learn how to "figure things out and think on their own"), make her tell you how she could have handled the situation different, and then tell her that you love her and hug her after you have punished her. Tell her you love her too much to allow her to be self destructive.
If we give them all the answers when they do wrong, just like if we were to do their homework for them, they don't learn anything for the next time the situation comes around. They need to not only figure out what they did and what they are being punished for but why and how they could have changed their outcome. They will remember it next time.
Unfortunately with the brushing the teeth thing you may have to go with her to the bathroom until it becomes second nature just like breathing and she does it automatically.
I hope this helps. I only have a 16 month old boy but lead and taught an all girls group for several years. From elementary to college.