Boyfriends sister

I need help. My boyfriend and I have a great relationship I get alon with his parents and extended family very well. But lately his younger sister (who is the same age as me) has been very rude and ignorant towards me. She makes it very uncomfortable for me when I go over to his house. And she just is always getting an attitude and is acting very sort of bitchy towards me and him (I bite my tongue a lot around her) my boyfriend tells me not to worry about it but I feel like in the future it will effect our relationship