I am jealous of my boyfriend's brother's girlfriend?

My boyfriend's brother and his girlfriend have been together for 6, going on 7 years. I came into the family about a year and a half ago and actually met my bf's bro's girlfriend prior to me even dating my boyfriend. I've had no issues with her until I started dating my boyfriend. She has been through a lot with their family, as you can tell 6 years is a long time. Because of this fact, my boyfriend has always called her a "sister" to him. Through witnessing multiple interactions between him and his brother's girlfriend, I can see that there is a level of comfort between them that I haven't established with the rest of his family like she has, and it makes me pretty jealous. She is also quite touchy with my boyfriend in a playful flirty manner, as she is with the rest of the cousins. I don't know what it is but I am just jealous of the fact that she has such a bond with my boyfriend that has grown for so long and I've only been in my boyfriend's life for 1.5 years. I've kept my distance from her but anytime he'll go out with his family and she's there, I just feel extremely uncomfortable. I've asked myself why can't I have the type of closeness that she has with my own boyfriend? This is really hard to explain but I'm hoping someone can help me out.
I am jealous of my boyfriend's brother's girlfriend?
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