Are you sure that this is indeed an IRA? If the account is truly an IRA, the financial institution is required to make the IRA/SEP/SIMPLE box on this form 1099-R. Having the box unmarked means that the financial institution is saying that the distribution was from some kind on non-IRA retirement account. The fact that "someone else contributed to it" has no bearing on whether or not the IRA/SEP/SIMPLE box needs to be marked.
