Taehyung knows it's wrong. Heck, its forbidden, but he can't help falling in love with his sister's fiancee. Especially not when Jungkook looks that tempting.
imagine if taehyung's last name was fück so it'd be fück taehyung but if he and jungkook marries jungkook's name would be fück jungkook and whenever people pass them they would be like " Look it's the fückers!".... sorry just a random thought.
As a frigging spaghetti. Just cook him tae, then he'll be all bent and squiggly