Here's my take on this subject:
Sang's tale is about a kid who lived in a bubble, unable to see any escape, yet desperate to find one. She has no experience with normal life because she's never HAD a normal life. The small glimpses into her early years show that normality never really existed for her. Yes, she had her grandfather to watch the baseball with, but not for long enough to teach her the other societal norms. For her EVERY relationship has been odd, because if you instil enough paranoia, hate, fear and distancing into a kid, there is NO way the poor kid can be normal.
Kayli's life WAS normal until her mother was gone, and she hates the "world" that inflicted her pretty ordinary life with a need to "take over" parenting when her surviving parent refuses to parent. She sacrifices almost everything to protect her brother's interests, not because she wants to, but because she HAS to.
Falling for more than one guy isn't as unusual as it seems on the surface: mothers do it all the time with their children. The capacity to love more than one person is NORMAL, it is society that decrees it ISN'T normal, and so patriarchal religions build on the idea that women should love their MAN, until they have sons, and only love the women in their lives if they are offspring or other blood/adoptee relations.
The truth is not so cut and dried. Humankind are built to love, and usually many people at once. So why is Kayli's story harder for some of us to read? Simply because for those people the CHARACTER is more confronting, because Kayli is NOT sweet, nor socially inept, or learning the basics about interpersonal relations. What she is is in many ways more normal, an extraordinary normal, but still normal. That means the story isn't about education in social matters, learning how to do relationships or how to change from bubble-land to living-land, but it is about a "ripping great yarn".