Why? They are like the main ship and all the story is built around the two of them so I don't really get it..
Because both siblings only have each other no one else, imagine if your sister slept with your partner even tho you don't love them, you'll feel betrayal, and beside the sister was mad when she found out about her brother wasn't because she's prejudice it's because her brother didn't tell her anything, after all she did raise him alone!
You spoke my mind. I absolutely agree with you.
I know what is the Sister like but I really hope that the Brother won't do that!