Nosy and instigating relative...help?

So my boyfriend and I have been together 6 years, he has a loving and close family, but his sister is nosy, manipulative and quite the instigator, She doesnt like me and feels I am not good enough for him, she even told him this, I am respectful and will not lower myself to her, I have yet to snap at her and tell her to back off, he did that for me....I just dont know what to do about this besides continue to ignore her and her stupidity. I plan on being with this man for a long time but it will make it hard for me to attend their gatherings if she is acting this way....advice needed....