It's a yaoi, a couple of chapters at least. The main character is asked to babysit his "nephew" a high school student, while his sister is away. He's never met the boy, I think it's his step-nephew and the sister is on her honeymoon with the boy's father.

When the uncle arrives the nephew makes some nasty comment about the uncle being gay (which he is, openly).

I remember a later scene when the nephew comes into the bathroom when the uncle is there and sees him with hickies. The nephew is disgusted and somehow the uncle forces him to have sex, not really sure how that happens because the uncle is the uke.

Any help is appreciated.

Found it! Koi to Karada no Kyoukaisen.