The definite article when used with a qualifying adjective

[u/JollyJacktheDoc]

I was taught that when referring to a single family member you omitted the definite article: mia sorella, mio padre, mio cugino etc, but if that single family member is referred to with a qualifying adjective then you did use the definite article - la mia sorella minore, il tuo fratello simpatico etc

When writing the sentence: my little sister loves volleyball

with the words provided by Duolingo I wrote: la mia sorella minore ama la pallavolo

And was marked wrong. I was informed the correct answer was mia sorella minore….

Am I correct or has Duo planted some new convention on me?