I'm on a sort of "Take Back My Life" rampage right now, and I've had several occasions to use this one on the ILs. The ILs do every major holiday at GMIL's. Every. Single. One. Since DH and I became a serious thing, I have gone to GMIL's for these "celebrations," disappointing my own family to keep the peace. Not doing that anymore. So, since I made that decision, I've been using this one a LOT on DH and my ILs.
DH: "I know you want to go to your mom's for Thanksgiving, but I really think we need to go to Grandma's, too."
Me: "I promised my mom she and I were going to spend the day together this year. Going to Grandma's just isn't possible."
DH: "But you could spend the morning with your mom and then come down with me for dinner!"
Me: "Cook dinner with mom and not eat with her and dad? That's just not going to happen. I promised mom, so going to see your family is just not possible."
DH: "But...but...the family is expecting us!"
Me: "I'm sorry, but I didn't make plans with them. I made plans with my family. I have a family too. Going down to Grandma's is Simply. Not. Possible."
Then he had to go and tell GMIL and FIL that I wasn't coming. So the phone calls have started from them. I swear they're talking from the same script. The only difference in the conversation is the octave their side is conducted in.
GMIL/FIL: "Why aren't you coming to Thanksgiving?"
Me: "I promised my mom she could have the whole day with me. I'm not breaking my promise to my mom."
GMIL/FIL: "But, we wanted the whole family together for Thanksgiving!"
Me: "So did my mom. What you're asking is not going to happen."
GMIL/FIL: "But this could be my last year with the family! My [insert disease/condition] is getting really bad!"
Me: "And you might remember that my mom spent as much time IN the hospital this year as out of it. I'm spending the holiday with my family. Coming down to see you will be impossible."
GMIL/FIL: "If you loved us, you'd come down!"
Me: *stifling laughter* "I'm sorry that you're disappointed, but it's not going to happen."
Then there's the whole "gift exchange," for Christmas, which we're bowing out of. They draw names at Thanksgiving and you get the person who's name you got a $50 gift card. I don't have $100 to spend on whoever's name I draw, plus GMIL and FIL (who insult you and call you cheap if their gift is less than $50) plus $20 each for all five cousins. That's $300, which is more than I spent on my family and my DH the last TWO years. I've explained that money's tight and we have to bow out of the gift exchange. They don't believe me. I'm just cheap
I've been careful to point out that we don't want people to give US things because we can't reciprocate in kind. It's not like we're gift-grabbing or anything like that.
IL: "You need to be in the gift exchange!"
Me: "That's not possible."
IL: "But, it's a tradition."
Me: "A tradition we can't afford. Please don't put our names in the Thanksgiving Draw."
IL: "But you have to!"
Me: "We can't. It's just not possible."
I wouldn't say that it's working 100%, but I've explained the why's once to them, and now it's just a merry-go-round of "you have to!" followed by "that's not possible." But at least I have a nice, easy script to follow now. Eventually they just give up and stop talking to me. Which makes me happy.