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.