You do not say how far away the wedding is.
For me though if you can fly out do the wedding and come straight back so 3 days tops then you should do so.
Assuming of course the baby is fine and your wife has support at home of some sort.
IT's your brother's wedding, you're the best man. Yes it will be tough on your wife, but it's not fair for her to use emotional blackmail on you. If you want to go, go. Don't take the mick and stay for weeks, but go there, do your bit, and come back. I'd be astounded if she were to have issues with that. I'm assuming the wedding date was planned before he knew you were expecting a child?
We can assume all we like, I asked the questions in this thread twice, the OP has been back into the thread and replied and gave absolutely no further info.
When was the wedding arranged?
How long until the wedding?
He has said Cyprus with daily flights.
So he can be there and back the day after the wedding, one night away, or two at most if he wants. I don't see the issue if this wedding has been arranged for ages. If however the brother decided a couple of weeks ago that he's going to get married, that is somewhat different.
OP also said s parents already live in Cyprus, so he can drop in and out of accommodation easily.