A small shiver ran up Kaoru’s spine as he felt those soft lips graze his ear. “I— I don’t know if I would be any good…” he murmured, lowering his eyelids as he looked away demurely, playing the part of the shy and insecure younger brother perfectly. “You would have to— to tell me exactly what to do… onii-chan.”
"Aaaah~? With you, brother dear? I thought I was your toy.” His lips put on an identical grin to the one his twin wore as he raised an eyebrow.
"What do you want me to do about? I’m just as bored as you.”
[…] After that, the girl said… [I’m sorry. Don’t cry, okay?] I remember how surprised I was when I heard those words. We probably looked like we were about to cry at that time.
Or if you prefer the English dub,
Shiro: And what’s worse, they’re twins, so that makes this totally insectuous!
Tamaki: I think the word you’re looking for is incestuous.