Phone your nephew and explain that neither of you are able any longer to keep going until midnight. Ask if he and his bride would mind if you came to the service and part of the wedding breakfast, leaving around 7 or 8 p.m. to drive home.
If he and his fiancée are all right with this, you will admittedly still have the time from the end of the wedding ceremony to the start of the breakfast.
I don't know how long that gap is, it depends very much on whether they are marrying before the registrar, or in church, where the length of the ceremony depends on the church.
As far as I recall most Protestant wedding take half to three-quarters of an hour, where a full Catholic nuptial mass takes around an hour and a half - I think mine lasted a full two hours.
If you can't afford the expense of a hotel room for a short rest before during the time-gap, you could, if the wedding is on a Saturday or a week-day go to the reading room of the nearest public library and sit there. It will be quiet there, or find a tea-room.
Are the no relatives of your nephew living near him, who might offer you house-room for a rest before the wedding breakfast?