Flintlock: The Siege of Dawn

The Cerith family saga is a series of two Sapper side-quests in the game. First, complete "In Safe Hands" which is given by Luca in Wanderer's Rest, then complete "The Family Condition" which is given by Johara after completing the previous quest and unlocking the Wisdom's Edge hamlet. These quests are pointed out in the guide linked in the Road Map. All quests in this game are extremely linear and everything you need is pointed out on the map.