your sister.. is more different than you.. she is a vampire.. now your parents.. are not even vampire so your mom must've got with a vampire somehow and got pregnant.. for your like she's been your younger sister.. she will always bug and whine to bite you.. because she managed to taste your blood.. and now want it..

you laying down on your bed.. then.. you feel someone breathing on your neck

Hey big bro.. can I bite you pwease!