Harrassment under landlord/tenant law (Protection from Eviction Act, 1977) is different to other types of harrassment, you do not need two incidents.
Harassment can include anything which prevents you living safely and peacefully in your home. It is:
An action by a landlord or someone acting on their behalf
It is likely to interfere with the peace or comfort of someone who has a legal right to live in a property.
It breaches your right to ?quiet enjoyment?, which means the right to live peacefully in your home. If anyone interferes with this right they could be committing a criminal offence.
Examples of Harassment (I have highlighted ones experienced by OP's SIL)
Entering your home when you are not there or without your permission
Persistently asking you to leave
Threatening you to make you leave, or offering you money to leave
Removing or restricting utility supplies such as water, gas or electricity supplies or failing to pay bills so that they are eventually disconnected
Forcing you to sign agreements which are designed to reduce your rights
Allowing the property to fall into such a bad state of repair that it becomes uncomfortable or even dangerous to live in
Refusing to let you into certain parts of your home, or letting you in, or letting you use facilities only at certain times
Harassment because of your race, sex or sexuality.