I just married Maru, on Summer 27 of Year 1. When we got engaged, I went to talk with Robin and Demetrius, and neither of them said anything different from what they always say. I have a medium level of hearts with both of them, and they're perfectly friendly, but they didn't seem to notice or care that we were engaged. They did attend the wedding, but they were just in the crowd of villagers; they didn't seem to be up front or anything. I guess maybe Pelican Town has different ways.