is taking tuition classes at home illegal

I stay in a two floor apartment with six flats. My mother has been taking tuition classes in our flat, which is our own flat from around 3 years for children in 2nd grade to 8th grade. All measures have taken so the other flat residents do not get disturbed. Our neighbour, is now harassing us to stop these classes saying it is illegal and it is a commercial activity. I would like to know that if these classes are illegal (considering that everyone these days take small classes in their homes and also that the income from these classes is of utmost importance to us). Also, what measures can be taken against his threatenings? Most of the time we are only ladies at home, as my father goes to work early and comes only at night, and the neighbour is really harassing us and we do not want to live in fear.