I'm sorry to hear about your sister's health challenges. My family has been through some similar challenges and I understand how fatiguing it can be to fight an ongoing battle against life threatening illnesses even beyond the point where you're sent home from a hospital.
But, her health challenges don't change anything regarding the landlord's legal rights to enter his property for maintenance, repairs, inspections or other valid reasons.
The landlord is required to provide 24 hours notice before entering, but is always allowed to knock and request permission to enter on shorter notice. From your description it does not sound like the landlord entered the dwelling unit, so he did not violate any laws. Sure, pounding on the door for 20 minutes was a jerk move, but that doesn't make it illegal.
So there isn't really anything for your sister to do at this point. If he asks about her sickness again she can either tell him the truth - that she's battling cancer and the rest of the illnesses are complications of her treatment, or she can tell him to mind his own business because she has a legal right to privacy regarding her medial situation.
If he comes knocking again, she can let him in if she wants to. Or she can tell him to give the proper notice and come back. If he does provide 24 hour notice, she cannot deny him entry - her illness doesn't change this.