DH "yes we're still going, if you guys ride with us maybe you could help with gas".
To me, this says that it had not yet been agreed for them to ride with you... the 'IF' says it all...
Personally... if I already had three people, and all of their luggage in my vehicle, then I would not offer to drive them and their luggage as well... unless you drive some huge vehicle.. I really don't see how it would be work out well.
We just got back from a weekend at the mountains yesterday... Just one weekend... our luggage, a cooler, etc... I see NO way that two other people and their luggage would have been feasible unless we had somebody hog-tied up on a luggage rack!!!
Not to mention that you will be literally and physically joined at the hip.. and if they do not have a car, they will be wanting to use your vehicle, or wanting you to drive them here, drive them there, etc...
Personally, for me.. It would never have been 'would you like to come with us'... It would have been "would you like to join us".
I don't care what all the previous posters have said about never 'asking'...
Times are different now.. gasoline is a huge expense!!!! I think that it is perfectly fine to bring up the subject of this expense. If fuel costs, for example.. end up being $200.00... a casual 'here's a $20.00' while at the gas pump is a joke and an insult. If they are eager to inconvenience you so that they can have a free ride... That is inappropriate and rude.
The bottom line is that this is your husbands sister/brother... Let him handle it, and accept it however it works out. 