As you selected to side with your real family in Hoshido, your goal to repel the Nohr invaders. Felicia, your personal maid, (or Jakob, your female avatar's butler) will arrive just in time as a reinforcement. Take note that you can now take control of Ryoma so it is advisable to move him and regroup with your main force in the bridge. Create a chokepoint by placing Ryoma up front and then moving Sakura and Azura closely behind. You can also have Takumi pair up with Ryoma to further increase his stats.
Elise and Leo will come to Xander's aid so be careful not to overextend your attack. Elise can be easily defeated by Hinoka since she'll be within reach and is the best target that can be defeated in an instant. Ryoma can also one-hit KO Leo. Once done, Xander will be left alone. You can have Azura sing to Ryoma to give him an extra turn then use it to defeat Xander. The quest will be complete afterward.
After the scene, Lilith will take you and Felicia to the Astral Plane. She will also offer you a tutorial about the game's new Castle feature. Follow this link to reach the Castle tutorial page.