I would be completely straightforward with your in-laws. Explain that you understand that they might be upset if you intentionally deliver when they're out of town but that you have to make a decision based not on your convenience or theirs but on what's best for your baby. If your doctor feels that it is best to indice at 38 weeks, because of concerns about such a fast delivery that it wouldn't be safest for you or baby, then that's what's best.
Maybe there's something you could do for them--like promise they'll be the first ones you call or something special--or maybe there's something special they could do for you--like bringing you a welcome-home meal after you get home from the hospital--that would make it feel special for them. But I think they will understand that you're doing what's best for baby, not for them. Just be honest!