My (33M) fianc? (30F) has been faking celiac disease for the duration of our relationship (19 months)

UPDATE: I tried to as gently as I could bring up my fianc?'s sister just to like, see if I could offer support or something and she absolutely flipped out. She claims to have never had a sister, she's an only child, and that her mother has "gone too far this time" to "invalidate my needs". She got super mad and stormed out of my apartment.
I just stood there kinda stunned while she drove off, I don't know what to do now. I feel like I just stumbled into some kind of psychotic family rabbit hole.
TLDR: My future mother in law revealed that my fianc? does not actually have celiac disease and does not need gluten free food, despite my fianc?'s insistence.
I've been engaged to my fianc? for the last 5 of our 19 month relationship. Throughout the relationship, she's always required gluten free food because of a celiac disease that causes many gastro problems for her if she unwittingly eats gluten. This has never been a problem for me, as I've always wanted what's best for her; even though it's a teensy bit more expensive I always buy the alternative food that she can eat.
Last night, we went to her parents' home for dinner, something we do maybe once a month or so. Her mother made spaghetti, a favorite of the both of us. We both ate a bunch and laughed and chatted throughout the meal.
I volunteered to take up our plates and brought them to the kitchen, where I noticed my future mother in law left the empty spaghetti box on the counter. I noticed immediately that it wasn't gluten free.
I ran back to the dining room to warn my fianc? that she will probably have "food issues" tonight, and she just kinda... Stared at me? Mortified?
After a pause, her mother flew into a rage. She started yelling at my fianc? about "telling that same old lie you'd tell as a child" and that "I'm disappointed you haven't grown up yet" and my fianc? ran out of the room crying.
I stayed with her mother for a few moments later and found out that it was actually my fianc?'s dead sister (!!??!?) who had celiac disease, and that's not even what killed her (I didn't ask what her actual cause of death was, I was too stunned.)
I went back to my fianc? who cried to me and said her mother never believed her about her celiac disease and that her mother is the one who is lying. I have not yet told her what her mother said about her sister yet... As far as I know, my fianc? doesn't even know that I know her sister existed. I sense that she is lying, but she's sobbing so hard about it that I don't think I have the strength to call her out on it.
What should I do?


Source.