It's for family. I wouldn't accept pay even if they offered.
I'm with you on that. I've viewed it similarly also when it was for friends. I've seen that many people on this site seem to cringe over that, but I think there's another way to see it when it's for family or friends.
....they have told me absolutely nothing yet about anything other than ''play at our wedding''.
Is classical repertoire even fitting for a wedding? It's in two months and I'm busy with large pieces of music and I don't have time to learn a bunch of 'wedding music'.
It would be good to speak with them and find out what they have in mind. It's possible that they're expecting you to play "Here Comes the Bride" and Mendelssohn's Wedding March, without having said anything. Maybe they're figuring "of course he knows we'll want that" and "of course he can play them." I've done a couple of weddings where it was like that, and it's a good thing I checked it out in advance, because before that, I didn't know how to play them.
If that is part of what they're expecting, and if you can't already just play them, I think it would be good to learn them, maybe even do your own little arrangements of them. That's what I did, and I made them a little more interesting than the standard thing, mostly for myself because otherwise I wouldn't have been that interested to spend time on them at all.
About playing classical stuff (not that those things aren't classical -- they are! -- but y'know what I mean)
.....maybe that's part of what they're expecting too. If not, and if you feel like doing some of your regular stuff too, maybe you could suggest it, and they might like the idea. It would then just be a thing of finding where a good spot for it would be during the wedding or reception or whatever -- and sometimes there isn't.
....(My first teacher had one odd request for a Chopin polonaise and the Fantasie-Impromptu....
Those happen to be part of what I played at 'my' first wedding. They didn't particularly want the standard stuff, but just whatever I wanted. I played the theme of the A-flat Polonaise for when the bride came in, I don't remember what I played instead of the usual Wedding March, and the Fantaisie-Impromptu and some other things were later on. They loved it. So did I.