["My youngest sister is about to start her senior year of high school.  I'm so proud of everything she has accomplished.", "Oh that's awesome! I am an only child but I bet it's so cool to watch your siblings grow up like that.", "Yeah, she's adopted and has been with the family for 14 years now.  I always try and help her study when I get the chance, and I'm excited for this new stage of her life."]
That is amazing and so nice of you!
When I was 15, my family adopted a little sister.  She's just about to start her last year of high school, and it's been really cool to see her grow up into a young woman.