I am also getting the docs for my in-laws, do I need to turn around and mail them to my in-laws?
I would keep your In-law's docs and bring them with you. They'll have plenty of time to fill them out the night before...it only takes about 5 minutes and none of the questions require you to look up information. The only thing they'll need is the credit card number they want to use for their sign and sail cards (if they're paying for purchases by card). My guess is that they'll have the card with them. Other than that it's name, address (ours were already filled out), how many times you've been to disney/cruised), are you travelling with any minors (are they yours) etc... Very basic stuff!
I'd be worried about them not getting to your in-laws on time if you mailed them...
Jess