"My sister is younger."

Translation: Η αδελφή μου είναι μικρότερη.

[kuchra]

Would "νεότερη" also work here?

[jaye16]

Yes, it is one of the accepted translations.

[Vivliothykarios]

My "η αδερφή μου είναι μικρότερη" was rejected in favor of αδελφή. Shouldn't it be okay, too?

[jaye16]

Yes, it was missing. It's been added. Thank you.