One observation: "Sobrinha" is niece and "sombrinha" is parasol, a little umbrella. :D
Haha, não fui o único que confundiu?... kkk... Errei bonito! Fui colocando umbrella sem ler atentamente. kkk
Why do I need eu?
You don't. " eu" can be omit
If I say I (only) have ONE niece, is it "UM"? "One niece" was rejected
No, you still use "uma".
Here, Duolingo accepts the correct "I have a ......". Two questions ago, "I have ......" was rejected and only "I have got ......" was accepted. Again, the irritating Duolingo confusion raises its timewasting head!