I was really worried about telling MIL when I had gone vegan, but she's fine with it. I just turn up armed with food - soy milk, nuttelex, tahini, nooch, and she sits back and lets me make myself at home in her kitchen.
Geez can I borrow ur MIL?? Mine took great pride in 'hiding' things in the food she prepared for me until I just stopped eating at her house. She won't even let her own (vegan) daughter prepare food in her kitchen so my chances are less than zero! It really gets me irate that she insists on serving DP "real food" (ie:meat & chicken) whenever we go there for dinner even though he has repeatedly told her of his dietary changes. Of course, if he eats it she will keep serving it to him, but it still frustrates me no end. Oh, and the best bit? She doesn't eat red meat herself and cooks her own vegetarian meals ... but is so passive aggressive about the dietary decisions of her DD & partner & girls & DS & I.

Phew, u really got me going there snugglybean ... sorry for the vent (but truly I will swap u MILs!!)