Typically, people who attend weddings dress festively, as weddings are usually celebratory occasions. But that Estele arrives “dressed as if she is in mourning” suggests that Adeline’s wedding is not something worth celebrating. Instead, it’s an occasion of “mourning” because it marks the end of what little freedom and independence Adeline has. Finally, Adeline’s father and Estele are likely right to be suspicious of Adeline, who clearly is lying about the present. The reader doesn’t yet know what Adeline really plans to do, but given the novel’s opening scene (Adeline running into the woods, away from the wedding party), it’s reasonable to guess that she’s going to try to escape.