How to Find D's Twin Brother and Complete His Quest: To Ds

First Meeting - Siofra River: First off, you'll need to head to the Siofra River. Make your way towards the northern cliffs. Look for a spot where you can drop down onto a ledge. This path will lead you to the Aqueduct-Facing Cliffs Site of Grace. From there, just keep following the path. You'll need to drop down again near an arch and land on a platform below. Continue heading north along this path until you see a waterfall. Turn right there, and you should find D's Twin Brother chilling near a balcony. He won't say much at first, but you'll get a prompt to give him the Twinned Set. Once you do, he'll teach you the Inner Order gesture.

Final Meeting - Deeproot Depths: For the next part, you'll need to have completed Fia's Questline all the way through to the Deeproot Depths. After you've got the Mending Rune of the Death-Prince, head to the nearest Site of Grace and rest to reset the area. Now, when you return, you'll find D's Twin Brother standing over Fia's body, looking pretty victorious. Talk to him until you've exhausted all his dialogue. After that, use the Site of Grace again to reset the area one last time. This is your chance to grab the Twinned Set back and, more importantly, the Inseparable Sword as your final reward for this questline.