"Non ho dei fratelli" is not ok but I don't quite understand why... Anyone care to give an explanation? :)
I think "dei" is used sort of like "some".
"Ho de mele", "I have (some) apples)"
You wouldn't say "some brothers".
Just a guess though ^^'
Is "Non ho niente fratelli" incorrect?
That could mean: " I have no brothers at all." if it were acceptable. But if you would want to say that >> Non ho nessun fratello.