It's possible that they're untrue, but the rest of the family wouldn't know.
It's also worth pointing out that many people with mental health challenges as adults experienced sexual or other abuse as a child. So even if she does have mental health issues now, that doesn't mean that her claims are untrue.
They're not just saying they don't know of any abuse. They're saying she has a history of trying to extort relatives by threatening to make accusations of sexual abuse.
It's not uncommon for people to stay quiet about their abuse until adulthood. Also very unfortunately common for people especially family members of the accused to not believe them.
And would someone please explain to me why this isn't a police matter if it involves rape allegations...
They don't do anythinhg about it. Literally the clearance rate of rape cases is less than 20%. Your only recource to punish someone for the harm caused is civil court.
Exactly how? They article gives almost zero information surrounding the allegations. They just immediately accused the woman of making up the allegations.