Board Thread:FAQ Board/@comment-27616769-20160408010316/@comment-25074378-20160514120943

She had a crush on Kazuto before he entered SAO, when she overheard that they weren't actually siblings, but only realized that it was a crush while he was in the game. When he woke up from the game, she found out that he was in love with Asuna, figuring that he felt that way about her due to the game. That's when she secretly began playing ALO to try to understand what he went through, eventually using it as an escape from her feelings. She eventually met Kirito in the game, unknowing that it was actually Kazuto (because she never knew his in-game persona prior) and developed the same feelings for him due to it being the same person.

As to WHY she allowed herself to develop such feelings for a cousin, well, the laws on marriage in Japan are more lenient with familial relationships, depending on the circumstances. Brothers and sisters are right out, but second cousins are completely fine, no questions asked. But with first cousins it gets complicated, and I'm not sure I fully understand it. It goes something like if both sets of parents have a sibling relationship between the two and both children come from that set of siblings, then it's not alright. But if the child is brought into the family through marriage, then it's fine. I think the situation with Kazuto was that his mother had him then married what would have been Suguha's uncle when both Suguha and Kazuto were very young, then Kazuto's parents died in a car crash (I think?), and so Suguha's parents adopted Kazuto in the aftermath. I'm not entirely sure about the situation, it's only vaguely touched upon and it has been awhile since I read it. The only thing that I can say for certain is that Suguha figured it out and it bypasses the legality issue.