I’d see no qualm in attending either way. That your nephew needs a dispensation is not your concern because you cannot obtain it for him. I’d also be cautious in asking about the dispensation because it may be seen as unwelcome butting-in, depending on your relationship with him. Perhaps bring it up casually and indirectly - “Oh, I remember our marriage prep - we had some really great discussions. Do you guys have a marriage prep through her church, too? Are you enjoying it?” etc.