Yikes! Four small children plus a newborn on a plane?
If it were me, I'd say thank you, but I'll see you the next time you and your new wife are in our area. A lot of people say babies under 2 or 3 months shouldn't fly unless it's a medical emergency (flying to get to a hospital which treats rare disorders etc.) Not to mention your recovering after your 5th childbirth.
Planes, with their recycled air are germy places, I know I always end up getting sick after I fly. Plus, the challenges of changing diapers in an airplane bathroom, keeping all 5 kids happy and secure on a long flight, the problem with ears popping in babies and toddlers, getting everybody through security... it wouldn't be something I'd do.
Personally, I'm not a fan of "destination weddings" in the first place. I think it's a huge imposition on people and It puts a lot of burden on the "guests" to pay for airfare and lodging and honestly, I think it's kind of unnecessary.
My dh and I always turn down invitations to such weddings. Too pricey, and too much of an interruption in our lives. It's hard enough to go on vacation every few years to a place we chose, pay for it, get all the kids settled etc. Being put in a position where we're kind of obligated to go somewhere we didn't choose, then be expected to purchase a wedding gift, get all dolled up to go to the actual wedding in a strange place... well, I'm not much of a traveler in the first place. But, I wouldn't go at all, and certainly not with 5 kids, including a newborn baby with a developing immune system, working on getting breastfeeding working for that baby and recovering from a birth.
I'd talk to your Ped and OB or midwife or Family doctor about the wisdom of taking not only taking a newborn on a plane trip, but also your health while recovering from your 5th childbirth.
Mileage and all that.