Check being held for fraud for no apparent reason?

I’m just a little confused. I understand with checks being deposited, that there are security questions that they have to ask to verify your identity and so on. But in this specific scenario, a check with gifted to pay medical bills and rent/utilities to my sister for her having cancer. When trying to deposit said check, they originally told her they would hold it for a few days, which is understandable. Then the fraud department called and started drilling her on very personal questions, including what kind of cancer she had, and why she was donated the money in the first place, and even somewhat shaming her for being donated said money. It’s now been under review for over a week, and all associated accounts are frozen and can no longer be accessed, and they’re also threatening to completely close said accounts. I’m more confused because they’ve stated that they haven’t even called the person who wrote out the check to confirm with them either. Is there anything we can do, or do we just lose the fight, they shred the check and close all of the accounts?