You should review this situation with an attorney who can evaluate the language of the divorce decree in detail. The short answer is: technically it's not an inheritance, since the asset did not pass by either will or intestate succession. However, under the circumstances, especially with the mother dying a week after the transaction, if you challenge it,  a court might consider this either a de facto inheritance (looking to substance, not form) or consider that this transfer was essentially a fraud on the court--an attempt to get around the court's decree. In either event, a court might use its equitable power to give you 1/2 of the money in the account . . . again you need to consult with an attorney, who can evaluate the language of the decree in detail, but this may be worth fighting, notwithstanding that on its face, this is not an inheritance.