Just becuase people get older does not mean they get mature or less self centered. Some people have never learned to care more about others than themselves. And it hapens with moms all the time becuase they get to run the family and make the plans. When their kids grow up and make their own plans these ladies, now MILs suddenly have to deal with plans that are not made for their convenience. I've learned that age seems to concentrate our personalities - so if we were caring and sweet as a young person we'll be more caring and sweet as we age.
Your MIL is obviously self-centered. Don't take it personally - you are the thing she has to go through to get what she wants - her grandkids! So she's doing what she's always done - made plans that are based on her wants & desires regardless of what's best for others, namely you and your kids.
For Pete's sake - you've had two serious car accidents and you don't want to get stuck in a snowy situation with small children. Sounds like you're being responsible. If she really wants to see the grandkids on Christmas ask her to come to your house. Throw a ham or roast beef & some baked potatoes in the oven - and make some peas & carrots or another easy veggie. It would be kind of nice to have someone over on Christmas - it will be nice for the kids to have a festive day too. And if you really don't want to socialize with her put the TV on - all the classic Christmas shows are on and you can all enjoy Clarence getting his wings, or Ralphie getting his eye shot out, etc.
When my husand was a rookie cop and had to work Christmas eve I still hosted his family at our house - it was my husband's childhood tradition to spend Christmas Eve with his grandparents and family and he wanted his kids to enjoy the same traditions. So although my childhood tradition was Christmas Day with family I honored his family and made a special time for my kids too. Now that my FIL is gone (this is our 2nd Christmas since he passed away) and my MIL is in a nursing home my husband and kids have these sweet memories and I'm so glad I had those Christmas Eve gatherings even though I wasn't thrilled about it at the time.
Try to be the more gracious person while still maintaining your boundaries - don't let her bully you and don't let her take your kids during this special season when moms and kids should be together.. Tell her you're going to be baking cookies together that day - or something like that.
It will be a good thing in the long run for you and your family to let your kids see you work around this selfish grandma. Kids learn by watching us - way more than hearing our advice. One day I'll be a MIL - and I pray I've learned how to be a good one by example of what NOT to do as much as what to do. ;o)
Good Luck Mama!