my gf's sister :-(

okay this is gonna sound stupid but it really bothers me that girlfriend's sister dislikes me. i know i shouldn't care but it kinda hurts. she plainly igonres me whenever im around her and everytime try to speak to her she just stares at me with this look of hatred in her eyes...it's the most hurtful thing. i don't get her she seemed okay with my gf and i being together but now it's like she hates me for who i am and the love i feel towards her sister. im not the only one who sees it. my best friend notices it and even my gf gets all nervous everytime her sister gets near us when we're together. i don't understand her. why should she hate me for being bi, i don't hate her for being straight