First off, LOTS of hugs to you for help in the major stress you're in right now. I 'm sorry that you're facing all of this wedding/family drama while your pregnant.
I don't know which came first, her choice of wedding date or your pregancy announcement, but only one of them can't be moved. If it is important to your sister that you (and your family) be at her wedding, she is the only one who has the option of making that possible by changing the date. Baby (and your doctor) will dictate when your pregnancy ends.
As for you and the newborn traveling at five weeks out, consult your physicians. Whether attending the wedding is what you want or not, health and safety needs to be priority. I had three C-section deliveries, and my doctor specifically said that I couldn't drive for four weeks or fly for twelve after each one. He recommended not flying for six months after the last one because, let's face it, everything gets weaker internally after being cut so many times. Cabin pressure also influences your internal organs. As for the newborn, our pediatrician said no to flying until they were three months due to general germ transmisison on the plan and ear development. Check with yours. Unless your mother or sister are physicians, I don't see where they can argue.
My youngest started Kindergarten this year as well. I wouldn't have wanted her to miss anything those first several weeks, not from an academic standpoint, but from a comfort, knowledge and routine standpoint. I think others have covered that though.
EMom said "For those who keep harping on how not attending will damage/change the relationship forever......Does it sound that peachy to begin with? Really? I get the feeling that if the OP knocks herself out, goes in debt, attends and works like a dog doing all the expected chores, etc., it still won't be appreciated. After all, they EXPECT her to do it. Why bother? If someone can't understand a $10,000 debt hanging over your head that your DH is working 2 jobs to pay off, a 5 week old newborn, a c-section that is still healing, not wanting to choose between missing your child's first day of school (Hell would freeze over before I'd miss my DD's first day of K.) or having that child miss the first week of kindergarten.......then they have pretty much ZERO empathy and who gives a rat's hiney about their feelings anyway? Because they clearly do not give a rat's hiney about YOURS!" Hear, hear!
I'm so sorry that your family is putting you through this. It sounds like you, your DH and kids are your own family unit though. Make the decisions that are best for you overall!