Family Drama Before Our Wedding

Heres the situation...my fiance and i live in the city where his father and step mother live...now i moved here with him and had no one. So his step mom befriended me. She was great to talk to. Really great lady, and i trusted her with things from my past, both good and bad. Heres the situation...its now less than two months from stan and i`s wedding and she has made up all these awful things about me. She has said that ive spoken to her about stan and i`s bedroom relationship (which i never did, i dont even talk to my OWN mother about things like that) funny thing is, she is making up all these crazy things, like about toys and movies, if you get my drift...NONE of which is true. Stan knows its not true, cause, well, he lives the life with me, and we have not experimented with those sorts of things ever. So anyway, shes dragging us into this crazy world of lies that she has created, which of course, makes stan wonder ( a tiny bit ) if its all true. Which we both know is not, but where in the world would she come up with these things? a split personality perhaps? anyway, its just driving me crazy that its supposed to be our time to enjoy planning a wedding, but all thats been happening, is this drama that surrounds this woman. If theres no drama, she creates it. Also, one other thing happened that she is angry about...when we sent out invitations, one accidentally got sent to her brother, who is on her crap list one day and not the next...which also means, she was on the guest list, than crossed off, than written back on, than crossed off. I just couldnt keep up with who was on her ungrateful list, who was on her grateful list, and who was on her rapidly approaching grateful list. So the invitation got sent, and at that time, he was apparently back on her ungrateful list...so not only was she mad at me for all that, now she is making up all this other stuff. Its making life really difficult. We live only a mile from his father and step mother, which isnt the problem. The problem is that now we have no relationship with them. This is all so crazy,and completely unexpected. Help me! what do i say next week when we all meet up and "discuss" things...