In Elden Ring, you’ll find a wide range of characters that will share their plight with you, most of these will have Quests that span multiple locations. This page will detail where you can find D’s Twin Brother and complete his Questline.
First Meeting - Siofra River
D’s Twin Brother is a curious character and rather than being involved in his own Quest, it’s more like an extension of D, Hunter of the Dead’s and Fia’s Quests. As you progress and complete Sorcerer Rogier’s Quest, he will hand you Rogier’s Letter, detailing D’s twin brother and where you may be able to locate him. You’ll want to make sure you’ve seen D’s own Quest through to the conclusion so that you have the Twinned Set on hand (although do not alter it).
Make your way to Siofra River and head to the northern cliffs to find a ledge you can drop down to, leading you to the Aqueduct-Facing Cliffs Site of Grace. From here, follow the path, drop down to the arch and then land on the platform below. Follow this path all the way to the north where you’ll spot a waterfall, turn right here and you’ll find D’s Twin Brother sitting near the balcony. He won’t say anything, but you’ll be prompted to hand over the Twinned Set, after which he’ll mumble and teach you the Inner Order gesture.
Final Meeting - Deeproot Depths
You must now see Fia’s Quest through to its conclusion in the Deeproot Depths. Once complete, and you’ve obtained the Mending Rune of the Death-Prince, rest at the nearby Site of Grace to refresh the area. You’ll now find D’s Twin Brother, donned in the Twinned Set, standing over the lifeless Fia declaring his victory. Exhaust his dialogue and then use the Site of Grace to refresh the area one final time and you’ll be able to reacquire the Twinned Set and the Inseparable Sword as a final reward of this Questline.