Good question. You concern is valid. I say safety first, if you don't feel like risking it then stay home. Your bf may or may not agree and choose to stay with your or go himself. That's his choice.

What about:
"Hey love, I know the party is important and I want to be there with you and your family but I'm not comfortable driving that far in the blizzard. I'm really worried we could get hurt. I'm going to stay home."

Something like that. Gently and respectfully indicates your needs. And doesn't critique him or his family. Doing that would likely trigger hurt or an argument so I recommend just keeping it simple and talking from your own perspective.

Good luck and stay warm!