My take on it is simplistic. I have two choices: Grow old peacefully but lonely, or grow old compromised and tormented but emotionally enriched. In other words: your heart will break or turn to stone; either way you're dying alone. Yes, very binary and cynical. But at least it rhymes.

Consider moving to Benton County, Oregon if you need a few extra weeks to meditate on the whole wedding thing. Remember: you're probably going to have to be there when this person dies. You'll also probably have to support their parents once they become so old they can't take care of themselves, and that will be in addition to the burden of caring for your own parents. Everything they secretly hate about themselves will come up once you have kids and they blame your side of the family for its manifestation in your offspring. From what I can tell, this is really difficult stuff, and I don't know 0.000000001% of it.

At the ceremony, ministers/Holy Men/Whathaveyou talk about god and faith and how hard it is to stay married in the abstract but they never talk about the specific problems of the two people involved. If they'd drop the pretense, I'd be more supportive of the whole concept. So she needs to let go of the fact that he slept with her sister. He needs to be more empathetic to her career ambitions. Make it more personal and less ceremonial. Let everyone in earshot know that they should hold these two people accountable to the outcomes of the compromises involved.

I say all this because I'm as tired of marriages as I am of divorces. Say what you're going to do. Do what you're going to say. Sounds easy, right?