Am I at fault if we keep this scandal a family secret?

I do not want to be in trouble for not contacting proper authorities, but am trying to resolve a financial exploitation matter in the family. I’m trying to get to the bottom of this before allegations of exploitation of an elderly parent are reported. My parent has had a child on her checking, savings and other accounts. The child has transferred funds over a period of time. He has not give any access to her account by other family members when asked after suspicious behavior has occured.
He has transferred money out to his name and sometimes money back in. There is a huge lack of funds and will be a nightmare to figure out. He has just closed her savings and said he created a hidden account to evade paying lawyers and probate expenses. He has only ok’d this with one sibling and the rest were not told initially of this hidden account. Then when other siblings and I found out, they were not in agreement and said put it back in her name. Anyway, the account money has not been transferred back, but we finally got access. Also, there are a number of online transfers into his account over the months from her account into his. His money was combined in the account. He says only a portion of it is our parent and it is untrue. My parent, who has dementia, and is getting worse, has no clue of this activity. she should have tons of money after 10 years and there’s not much left.
Can anyone help me out here?