In any case, it won’t be far from the settlement where you first meet Radagos. The hideout will be added to your tracker automatically, so it will be easy to find.
The dialogue is fairly linear and will inevitably result in you agreeing to meet Radagos at a nearby hideout, where Galter is located. When you speak with Radagos he’ll tell you that he knows where your siblings are and that he’s willing to show you under one condition: that you allow him to execute Galter, the slaver who kidnapped your family.
When you visit a settlement after completing Rebuild Your Clan, meeting Radagos will be the first thing which occurs before you can do anything else at the settlement, making the quest impossible to miss.