User blog comment:Jonnylegoboy911/Was Túrin Turambar actually blind?/@comment-207.250.184.202-20160506152938

I think that when Túrin described his sister, he was actually describing his other sister, Urwen, also known as Lalaith. She died in a plague when she was only a few years old. But Túrin never met his other sister, Nienor, until the whole thing where he found her, fell in love with her, and married her. So when Mablung asks about his sister, he naturally remembers Lalaith, not Nienor.