How can someone who stole money from a relative, and has repented to Allah with sincere repentance, but cannot currently return the stolen goods for fear of exposure, repent and maintain concealment while guaranteeing the restitution of rights?

If the stolen items are fungible (mithli), then their like must be returned. If they are non-fungible (qīmī), then their value must be returned. It is not a condition to inform the relative explicitly about the matter; rather, restitution can be made indirectly. If you are unable to return the stolen item, it remains a liability upon you until you are able to fulfill the obligation.