Gift tax

If my mom and I transferred 16,000,00 from her name to my name 6 years ago.  And she just passed away. I have 3 siblings. I am supposed to split the 16,000.00 four ways.  Os there gift tax due?  Was she supposed to pay it?  If she didn’t how would I know?  The year she transferred the money was 2016.  So I don’t believe it is inheritance. So it must be counted as a gift.  Being that it was 2,000.00 over the max annual gift and assuming she did not pay gift tax on that amount, do I owe it?  How do I proceed before dividing it up and paying my siblings their share?