Hmmm... what would you rather do? Other than the flying thing? Do you have fun with your BIL, SIL... Do you like sharing a hotel room like that?
Flying isn't a big deal. I don't especially like it. But, it's okay. You check in (usually have to show your license/passport - keep that handy because you'll need it again). You check your bags in - be prepared to pay for them - depends on the airline and how many/heavy your bags are. Then you go through the security checkpoint. They want to see your ticket and license. You have to put EVERYTHING on that belt scanner, including your shoes and then you walk through. After that you just go to your gate and sit down. Or eat. Or shop. Or whatever.
They will call you by rows and you get on, put your carry on under the seat in front of you or above you in the storage. Sit down, get buckled. Have a good flight
Going to Disney is always fun. And I never feel quite comfortable with my in-laws then I do with my own family. But, I don't especially like spending time with my family either