Adoption: Words don't fail me now!

Twenty one years ago my husband made a painful choice. He and his girlfriend placed (i don't know the correct or respectful term here! i've been online all morning trying to figure out the best way to word this) their daughter up for adoption. It was a terrible heartache for Chris and continues to be. HOWEVER, she, Heather (doesn't she have a beautiful name?) has been raised by such an amazing, generous, and loving family that we can't help but feel thankful. We have the good fortune to spend time with her every summer. It doesn't hurt that we live in California. If we lived in Minnesota it might be different (she knows i'm teasing...) She comes out and stays with us for about a week and we have a great time. Now here's the dilemma, I have no idea what to call her!!! Chris and I are under no delusion that she's our daughter. Obviously, I had nothing to do with her birth but we understand that we also had nothing to do with her being raised and we firmly feel that her parents (adoptive parents, chosen parents, or whatever the PC terminology is) ARE her parents. We would never want to take away from that or diminish it in any way! However, our boys think of her as their sister. Do you see my issue?! My husband is the birthfather, I'm the woman he's married to, and our children are her birthsiblings (?). We all love her with our whole hearts and miss her when she's gone back home. But what is she to all of us as a family unit? Without offending her or her parents or her sisters, what can we refer to her as?