[Maekawa Hayato] Onee-chan ga Doppelganger ni Netorarechau! ch.1-2 [desudesu]

Shouichi comes home to a sound no brother should hear through his sister's door — and the boy making Ryouka moan isn't a stranger, it's a perfect copy of himself, one that shares every sensation with the original. While the shameless double claims first Tomomi and then Ryouka with a confidence the real Shouichi never had, he can only watch from doorways and hallways, feeling every warm mouth and tight clench secondhand through their uncanny link. Just when he thinks he understands the theft, a dark-skinned duplicate of himself appears mid-act, leaving him to wonder how many versions of 'him' are out there claiming his sisters.