Don't discount that the wedding can be a renewal of vows for a couple already married. Maybe the chief and his wife get back together and renew their vows? I really don't think any of the couples on the show are ready for marriage at this point but who knows. And you know how some shows love to have parents visit their adult children, only to find out the parents are secretly divorced but didn't know how to tell the kids. Maybe Burke's parents are divorced and will be getting remarried. There are lots of possibilities.
And Shonda is very vague. "Involves main characters" could just mean they are in attendance but none actually getting married themselves. And "it will surprise a lot of people" could be a joke on us, that we'll be surprised when we find out it's not one of the main characters actually getting married after we spend all this time speculating on who it can be.