Be careful, CF.
It seems that you don't yet have proof that your DMIL said any of those things. It's not unknown for troublemakers to invent or embellish a story to cause trouble between two people. Try to get some proof before you say more in public - anyone in the world can read this thread and the laws of libel and slander apply to you too. If you want her to stop, confront her directly. If you want to humiliate her in revenge for what she may have done, get the proof first.