Why does everyone's mind instantly go to incest?

Why does everyone think that Yamato's sister is in love with Yamato? I think the awkward atmosphere has to do with the fact that they're both pretty awkward people. They also both have the same sort of lack of facial expression. She's just able to read him better since she's seen how differently he reacts to Kakeru.

Plus, I'm pretty sure she's also gay or at least had a crush on a female childhood friend. When I first read chapter 5, I thought she also had a crush on Kakeru. She said that her and her brother were the same. But after chapter 11, reading her talk made me realize that she probably was rejected by a female friend and lost that friendship. It's either that or she realized her sexual preferences early on and what difficulties she could potentially face in the future (ie falling for a friend). Seeing her reaction to being seeing as a couple with her brother, she was probably not only uncomfortable because it was her brother but it was probably a reminder to her that recognized couples are almost always boy/girl. She'd wouldn't get that comment if she went out with future girlfriend.