Family Bonds (兄妹の愛, Kyōdai no Ai lit. Love of Brother and Sister) is the first paralogue of Fire Emblem Heroes.
The Names of the three parts of this paralogue are Sacred Bond (聖魔の兄妹, Seima no Kyōdai lit. Brother and Sister of the Holy Demon), Holy War's Bond (聖戦の兄妹, Seisen no Kyōdai lit. Brother and Sister of the Holy War), and Unbreakable Bond (兄妹愛再び, Kyōdai Ai Futabi lit. Brother and Sister Love Once Again).
Part 1: Sacred Bond
Subjective: The following part of this article is based upon the editor's personal experiences and opinions, and therefore may not be applicable for all readers.
Whether you move your units one block away from Ephraim's attack range or exactly on his attack range, he will move forward in the Enemy Phase. If one of your units is a powerful Axe unit, then you have the advantage and you'll take him down at ease.
Then, move your powerful Red tome unit to defeat the Emblian Green Mage. But be warned, the Emblian Lance Cavalier will attack your Red Tome unit, so try to keep that unit alive. When your next Player Phase begins, defeat that enemy Lance Cavalier with either your powerful Axe unit, your Green Tome unit, or your Green Dragon unit.
Finally, when Eirika is close enough, defeat her with your powerful Lance unit, and you clear this stage.
Part 2: Holy War's Bond
Move your powerful Sword unit to the Emblian Axe Cavalier's attack range on the left side of the map and your unit gains the advantage to defeat him during the Enemy Phase. Then, send your powerful Red Tome unit to Julia's attack range and during the Enemy Phase, if your Red Tome unit is powerful enough, then you should be able to defeat her at ease.
Seliph and the Emblian Cleric will not move until their next Enemy Phase, so move your powerful Lance unit close to Seliph's attack range. When Seliph is one block away from your Lance unit, take him down. Also, defeat that enemy Cleric as soon as possible, or else that enemy will heal Seliph or she can attack your units, whichever comes first.
Part 3: Unbreakable Bond
You will have 4 units on your team. You will also have 4 bosses: Ephraim, Eirika, Seliph, and Julia.
If you have your powerful Lance unit, send that unit to Eirika's attack range so your units will defeat her. If you have your powerful Axe unit, send that unit to Ephraim's attack range and your unit will defeat him.
Then, if you have your powerful Red Tome unit, send that unit to Julia's attack range and your unit will take her down at ease. Finally, when Seliph is close enough, defeat him with your powerful Lance unit and you clear this stage and this entire Paralogue.