Cold Equations time.

If it is bad and they are that far away, write them off. You'll have more to do than worry about them, and after a week or so, they won't be showing up. Either they'll be out of gas and cash somewhere in the middle, dug in at home, or dead.

If they show up, great, celebrate. But unless you yourself can directly influence their ability to reach your location, there is nothing you can do. If they aren't within a few hundred miles, realistically they'll never get there it is very bad.

As for your European relatives, it there is a long term communication breakdown, they might as well be be in a different solar system.

I hate to sound mean or cold. But in a scenario so significant that there is a long term disruption of communication and trade, you can not help them. You can't change the weather either, only deal with it.