I feel like this needs some consideration. Now that we know, it feels very targeted and intentional that the author pointed out how although everyone calls them siblings, they've only known each other for something like a year. They effectively don't have a familial relationship aside from being informed they were cousins, so irregardless of whether or not it's acceptable or gross for her to I feel like it's just not that deep. She's a closeted teenager, suddenly this very cute girl (who is apparently her cousin) is very in her life, and she's crushing on her. I feel like the "you think it's disgusting don't you" angle can be for both the homosexuality and the familial connection personally.