The Family Blade is a Skellige side quest with suggested level 15.

You can pick this quest up from Kaer Trolde notice board. After picking up the quest, head towards the map marker, which is near the obelisk, and you'll come across the thieves.

After dealing with the thieves, loot the corpses until you find a journal entry. Read the journal entry to find out where the sword is now.

After reading the journal entry, head towards the next map marker, which is at the old fort. Once you near the old fort, kill the thieves there. The same thing again, you'll need to loot the corpses until you find another journal entry. Read the journal entry to find the next place to look for the sword.

The next location will be near the whale graveyard. As you near the whale graveyard, if you keep your distance you can hear the thieves talk about what they want to do with the sword, or just attack them. After dealing with them, loot the corpses to find the sword! Finally!

Return to Kael Trolde and to the guardsman named Olaf, and collect your reward.