Rose is bisexual, right? because i always read people calling her a lesbian but if im not wrong she had a crush on dave at some point (before knowing they are brothers ofc)

Yes she did use to flirt with Dave back in the day, but given their relationship there is a high likelihood that it was ironic. So believe what you like, bi, lesbian, pan, whatever the fuck is fine man. Homestuck is all about the headcanons.