It is distressing when our loved ones are not functioning at their best. Remember your mother is 93, she may be on a variety of medication which add to her paranoia/delusions. Act with love towards her. You have a variety of acceptable responses to her, from apologizing to confronting. Maybe look for a response in the middle. You could simply say, I am sorry you feel that way and change the subject or just change the subject. Focus on maintaining a positive relationship. Rather than "visiting" or spending your time talking, look for activities to do together, like playing games/cards or reading to her. She has a lot of time on her hands to think and her thinking may go astray. You don't want to spend your time on that. Just spend time with her, be present.