Tell only IF it is necessary - how much do you see them? Maybe divide and conquer? I like my SIL, we are friends. Not crazy about my MIL and neither is she. So we giggle together in corners. If you have to, remind yourself it will end, do these people and their opinions really matter and Christmas is just one day. My Father's funeral totally arranged by my step-mother and it was terrible for me, my sister held my hand and whispered "it's just one day." It's not your job to educate or change these people.