POF, you are being perfectly accommodating. More than considerate, actually.
Your SIL is being passive-aggressive, and you are handling her beautifully. "No, I'm sorry, I can't do that." Lather, rinse, repeat. You are being perfectly polite, in the face of rudeness.
Considering the information in your second post... I'd bet money the BIL will have lasagna anyway, or else he'll complain about the special portion you've made for him being tasteless. Be prepared for further passive-aggressive behavior, and remember to just smile and let it roll off your back.
Everyone else will have a good time, and that's all that matters.
Have a wonderful holiday! I'll bet that lasagna is fantastic; enjoy every bite!