Let it go. In some ways, yes, they took you because you were a legacy. But that's not bad. It's a good thing to want your legacies. I was a legacy, and I know they took me because my sister was in the chapter. I didn't think about it for a second. If anyone thought that I wouldn't have gotten in without my sister there first, well than too bad...I got in!!
Make the most of the opportunity and the sisterhood. They gave you the opportunity to be a sister to your family member who is already a part of their sorority. That is a gift.