Definitely let them know, or have your boyfriend give them a heads up. My in-laws constantly tease me about being veg*n, but they are so sweet about making sure I have enough to eat. I think the best way to approach it is just to let them know about your situation and offer to bring something to share with everyone. That way it doesn't put pressure on them, especially if they don't know what to feed you. If your boyfriend's mom does not want you to bring anything, just let her know that you'll be happy to eat a salad or side dishes. I never expect people to cater to me, but I'm often pleasantly surprised at how accommodating they are when you let them know ahead of time. I think she would probably be embarressed if you showed up to dinner unable to eat the meal and she didn't know about your veg*nism beforehand.
Let us know how it goes!