Mother in law umes that her daugter and granddaughter are in our wedding, will not listen!?

We are getting married next year, have already picked and informed our wedding party. Only two on each side and then OUR little girls as our flower girls and our son as the ring bearer. My mother in law has just umed that my fiancees sister...her daughter (who I do not even speak to unless it is a family thing...Christmas and Thanksgiving!) and the daughters little girl will be in the wedding. I have told her no, that the party has already been chosen and that our children are in the wedding. She is telling the rest of the extended family (that are not even invited, my fiancee has not seen them since he was like three...that her daughter and the girl are in our wedding. I have made it clear they are not. I am not trying to be mean, but even though it is his sister, they are far from close and we see the little girl twice a year. They are invited, but not in the wedding. The mother has told family and co workers of hers that they are and that she cannot wait to see them!