sisters adoptive family

So not too long ago I met my sister. And she's married and I have a niece. My nieces birthday is coming up this weekend and I can't go basically because of her adoptive family. Funny thing is to me that these people are supposed to be christians but can't accept me for something that happened that I couldn't control? My sister was adopted to them 10 years before I was born. They can't take that out on me or my other siblings. How is that fair? I'm so hurt. But I figured it would happen.