This is a saying I personally use a lot, because I have been teaching for over 10 years and just once I accepted the request of teaching a family member. It is like the reason why doctors do not treat their family member. We are too emotionally involved.
That is why, Brazilians usually say:
“Casa de ferrero, espeto de pau.”
We may work as teachers, but we do not always bring our work home. Have a look at the picture to understand the meaning of this expression: