Sibling: Should I tell my sister she's adopted?

Here's a 16-year-old sister with much more sense than her mother, who's been lying to the 10-year-old sister about her birth -- and an advice columnist who's got it right:
Dear Amy: I am a 16-year-old girl with a 13-year-old biological brother, "Paul," and a 10-year-old adopted sister, "Natty."

My parents have never told Natty she's adopted. She resembles our family, so I don't think she notices.

She recently started asking questions about her birth, and my mom has made up elaborate lies to cover up her adoption. My mom says she'll tell her when she's old enough. Paul and I think she deserves to know but don't want to defy my mom.

Can you help? — Distressed Sister

Dear Distressed: Your mother's refusal to tell your sister her adoption story has now devolved from lying by omission to outright lying.

Your mom is putting all of you in a terrible position, and it has the potential to profoundly affect everyone.

Your sister is old enough to learn her adoption story. She was always old enough to know this story, because it's the truth. It's nothing to be ashamed of or worried about, except, of course, when it becomes this big and powerful secret that the whole family must keep.
Just when you think things are getting better -- more openness, more understanding -- you get proof that adoptive parents can still be mired in the bad old days of adoption secrecy and shame.  We're talking about an adoption that happened only 10 years ago, not in the last century, not in the decades when social workers insisted that adoptive parents did best by keeping the adoption a secret.  Only 10 years ago. 

And do we need a better reminder that if we, as adoptive parents, shirk our responsibilities to talk openly and honestly with our kids about their adoptions, then someone else with step in to tell them.  And as sensible and well-intentioned as this 16-year-old big sister seems to be, do you really want her to be telling your child about her adoption?!