Hi Lac,
Firstly, if this person's grandmother is repeatedly contacting you to harass and threaten you, and you have asked them to stop and they have refused, it might be time to seek an AVO through the police. At the very least you can tell her that if she contacts you again or contacts your workplace that you will be getting a restraining order.
Secondly, if you have spoken to the real estate agent privately and truthfully about the state of the house and the behaviour of your housemate and additionally the real estate agent has seen the evidence of this with their own eyes, then your housemate and/or their grandmother won't be able to successfully sue you for defamation. My guess is that they are using empty threats in an attempt to intimidate you into doing what they want.
Thirdly, how did the real estate agent respond when they saw the state of the house/ room and you explained what had been going on? Have they agreed to help you get rid of your housemate? Also, have you told the real estate agent that you are being harassed by this person's grandmother? If the real estate agent isn't being helpful, then you may want to get in contact with your landlord. I'm sure that they would want to know if their property is being mistreated and your attempts to get rid of the offending tenant are not being assisted by the real estate agent.