I'm in the same boat. My sister is suffering from delusional disorder. She has fixed beliefs that family members and non-family members are "coming on to her" or her son.  Its a form of "erotomania" and "persecutory" delusions. The pattern began after she suffered a head injury from being hit by a car.  
She has accused my boyfriend, my paternal uncle, my aunt's colleague and my dad of sexually harassing her. She has accused my dad of molesting her and her son.  She has also insinuated that my MOM was inappropriate with her son.  It's a nightmare. I just try to be there for my parents.  I keep my distance from her. I don't want her accusing me of anything. I already feel angry and tormented enough for my other family members. I try to be there for her and focus on positive happy memories and connections. I don't know what else to do for her. I tried to help "fix" her but I can't. It feels like a living hell. She and we are trapped by it.