I always travel for family BBQs/parties and I have always offered to bring something. It could be something simple like drinks (soda/water), paper goods, pre-packaged rolls, cans of baked beans.
I think "I have to travel, how can I bring anything" is a lame excuse. You don't have to prepare any gourmet dishes and cart them along, you can stop at the store 10 minutes from your destination and grab something.
OP, since you weren't the one who came up with the BBQ idea, but are graciously hosting it for your family, there is nothing wrong with asking everyone to contribute.
I think "I have to travel, how can I bring anything" is a lame excuse. You don't have to prepare any gourmet dishes and cart them along, you can stop at the store 10 minutes from your destination and grab something.
OP, since you weren't the one who came up with the BBQ idea, but are graciously hosting it for your family, there is nothing wrong with asking everyone to contribute.