My wife and her sisters have a joint bank account and want to transfer their shares of it to their personal accounts. Would such transfers be reported to the IRS?
I am not an accountant in any way, but I can't imagine needing to report that to the IRS. It's not income and has probably already been taxed when it was earned/received. You're not moving it from an untaxed (Roth) retirement account to a taxed one, or vice versa.
Anything over a $10,000 deposit is reported to the IRS. I'm not sure if you need to fill out a form or if the bank does it automatically. As long as it's legit I don't see that there would be any issues. The worst that would happen is somebody would have to talk to the IRS and explain that it was their money and not a gift of some sort.
Edit it is reported to the FinCEN not the IRS