Well, I'm not answering your questions, just giving my opinions. So, you've saved & planned for this vacation with your DH & ILs for 3 years. You're "very close" to your sister & she doesn't ask for much? Are you not "very close" to your DH? Does he ask for too much? Do your sister's wants take precedence over your husband's?? Coz really, she just wants you there to share her happiness (which I understand), but she certainly doesn't
need you there in the first few days.
No wonder your DH gets sad & bickers. His preferences are taking a backseat to your sister's. I think you should "specifically" tell your sister that you have planned your trip for 3 years & there's no way you can disappoint your DH & son by postponing it for a year. Really, they should be your priority.
A previous poster said WDW will be there. But guess what, so will the baby. It's not going anywhere (for at least 18 years!
) Share your sister's joy now, throw her a great baby shower, shop with her for baby items & plan to spend time with them before & after your vacation. Your sister's family shouldn't take priority over your's. JMHO.