Didn't the sibling have to declare the amount of funds that were disbursed from the account after probate as income on her personal tax return in the year the income was received?
I believe the OP would have the same issue when/if he were to inherit the money in the jointly titled account.
This was a checking account that the parents had and the sibling name was added after both became ill and incapable of taking care of themselves. A power of attorney was obtained. Upon death of the last parent the account balance was included in the deceased's estate. Like any other state, the account (and other assets) were subject to the estate laws. After a few months the estate was closed and the assets distributed.