I have been in this position......and it's awful.
My brother was engaged when I got pregnant, and when we called to give him and his fiance our good news, they were pleased for about two days, until they called and asked us if we'd like them to arrange for a sitter for that weekend (the wedding was a 7 hour drive from us). I was 5 months pregnant. I said I didn't think I'd need a sitter; the baby would be at most 3 weeks old and I would likely just carry him/her the whole time, being sure to stand near the back during the ceremony so that the tiniest peep could be quickly ushered out of the sanctuary. I even offered to have my MIL, who was invited to the wedding, stay completely outside the synagogue with the baby during the ceremony, so that even the initial newborn squirmy hamster sounds wouldn't be heard.
That wasn't enough for them. They wanted no baby at the ceremony, the reception, the prenuptial dinner, and the pre-wedding photo session. The baby could not even be in the lobby of any of those places because, in the words of the bride to be, "I am not going to compete with a baby!"
We were devastated, but weak. We found a postpartum doula in the town where the wedding would take place and planned for me to run back and forth between the hotel room (where doula and baby would stay) and the wedding all night, even rented an additional room in the hotel where the prenuptial dinner would be so that our baby could be close by. My brother and SIL-to-be issued blow after blow, even kicking me out of the wedding party because I insisted that I would not wait 2 hours between visits to my child just because SIL2B heard that's how often babies nurse, and more frequent exits from the head table would be "distracting." The baby was "allowed" in public only the morning after their wedding, at brunch. The months approaching the wedding were a terrible time for my family. My parents were furious with my brother but felt they couldn't say anything. DH and I were terribly sad, and worried about being brand new parents under such circumstances. The phone calls from my brother and SIL2B got uglier every week. My blood pressure climbed dangerously high as a result of all the stress, and I honestly believe my labor and delivery were compromised in part by the terrible family politics.
As you can imagine, the wedding weekend was brutal. The long drive with a newborn, the constant nursing, the running back and forth, the feeling secretive, etc., was compounded by my brother and SIL doing nothing to help make it easier. While I was nursing the baby, my place at the table for the reception was given to SIL's grandfather's nurse, and I had to sit on a stool to eat my dinner. During the reception, SIL's grandmother asked me where my baby was, and when I told her the baby was upstairs in the room with a sitter, she said she couldn't believe I would leave such a new baby with someone else. Obviously, she didn't know her granddaughter very well. It was a nightmare.
13 months later, my brother and SIL were divorced. My brother called me, crying, regretting how horribly we were treated.
So, looking back, we made a mistake. Even if their marriage had lasted, we made a mistake. We should not have gone. We should have put our daughter first. We regret having gone to this wedding and having begun our daughter's life in such a negatively charged environment. I would recommend that you do not attend this wedding, either of you. I think you will regret the hassle involved, resent those who imposed it on you, and feel guilty for having participated. That's how we felt.
Given the chance to do it again, we would NOT attend. I would have gone back to that first day when they told me the baby was not invited, and I would have said, "ok, I understand. Thanks for being up front." Then I would have politely bowed out of the wedding party and told them we didn't feel comfortable making the trip that soon after our child's birth if we were going to be separated from the baby for so much of the weekend. No judgements, no arguing, just simple and firm. I would have sent not just a wedding gift, but a beautiful bouquet of flowers and wine to their room on their wedding night, since I'd be spared the cost of travel and hotel room by not going. Attached would have been a card that congratulated them, expressed regret at not being there to share it with them, and inviting them to visit as soon as they could. I'd have signed it from all three of us, and left it at that.
It would have caused some bad blood, but there ended up being bad blood anyway, and at least this way, I'd still have my dignity. I'm ashamed at the way I behaved as a new mother. I wouldn't do it again.
Just my two cents. Good luck to you, whatever you decide.