Any number of possibilities.
Look at the whole Emo thing that started back in the 80s. By 2000 it was common for kids in that subculture to be bisexual just to be ‘cool’.
Dunno, never knew much about that stuff. Did not see much of sister’s kids after they were 5-6 years old. Took my wife to clue me in when they were in their twenties and she first met them.