Rock and a hard place !!!!!

I am having the hardest time opening up to my boyfriends sister .. I just feel like she judges me and doesn't like me and looks down on me any time we are near.. I'm trying for his sake to talk to her and try to get to know her and she just shuts me out .. Well that bothers me bad bc I want to talk to him about the way she makes me feel and stuff but I don't want him to feel bad .. I keep trying to talk and she blocks me off.. And I'm naturally introverts so I'm really trying when we are near each other.. But yet I don't talk enough or joke .. But everyone knows if you open up you have messed up.. Advice please