My mother passed away and left my brother and I an IRA at 50% Beneficiaries each. We both received a lump sum amount- I know this IRA was started in 1991 and then rolled over in 1998 to an annuity paying more interest. In 1998 it was $16286.72 (upon her death it was $34,688.56), she never took a RMD. I also know she never made any contributions to this IRA other than the earned interest. I do not know and can not find out how much it was started with in 1991. Her past Tax returns show NO deduction for any IRA- how can I determine the Basis of this IRA and how can I determine the taxable amount of my inheritance? Thank you very much.

Since IRAs are funded with before-tax money, basis is irrelevant.

ALL of the IRA is taxable to the heir.