AIBU to be shocked at Mumsnet Thread

Neither should be believed or disbelieved until the issue has been properly investigated.

We have seen recently cases where the police have assumed people were guilty of sexual abuse because they believed that accusers always tell the truth and it has done much damage to innocent people - and cost the police a lot of money.

In a case like this where there are all kinds of other stresses in the family, both the child and the grandmother need protection until the issue has been properly investigated. It may be true, but on the other hand some children, like some warring parents, know that nothing gets everybody on their side faster than an accusation of sexual abuse

I would not rule out any possibility that an older female relative might be involved in sexual abuse. We do not know how old this grand mother might be, heart disease or not it happens.
The older woman is the alleged perpetrator. She could have been doing this for years which might explain the girls mental health issues.