Anonymous wrote:OP here. There is a little more to this. I found out because I arrived early to help MIL. I brought a few of the dishes. We both cook. I found out close to when everyone was arriving and about to sit down. MIL told me to say nothing and like a child I didn't say anything. Throughout the whole meal I kept wondering what to do. Should I interrupt everyone, out MIL and start WWW3? Should I pretend to trip and knock SIL's plate on the floor? If I did this though she would just get another serving and I'd have to keep doing it which would be even stranger. So I said and did nothing. Afterwards MIL, told me that she has been doing this for years. BIL/SIL have been married for over 10 years .
I really doubt that SIL knows. She is very passionate about being a vegetarian for environmental reasons. She doesn't cook so I don't think she makes any of these dishes on her own. I go out of my way to make real vegetarian dishes which she seems to appreciate. Her appreciation though turns into a lecture to everyone at the table about how we should be eating this instead of the meat. BIL "became" a vegetarian when they got married but sneaks meat in the kitchen at every family event or dinner. I'm sure he is ordering a streak at business conferences or work travel. She freaked out once when she caught him in the kitchen sneaking some of MIL's roast. I really doubt that she is politely eating MIL's food and pretending not to know it has meat.
I could tell DH and have him tell his brother but it will still come back to me. I will have both of them angry at me and I'll be in the middle. SIL will be furious that I didn't tell her that night and will be lecturing for the next 10 years. MIL will be furious that I outed her and I'll be hearing about that for the next 10 years. I love my husband so getting a divorce to escape these two is not going to work for me. If I stay silent, I know that I will be complicit in this fraud for every future family event. I'm sure that MIL pulls other crap on people. I really do not want to build a relationship with her where she confides or confesses anything else.
Regardless, you tell her.