how is nimphadora niece of bellatrix and narcisa????

her mother Andromeda is Narcissa's and Bellatrix's sister => Tonks is their niece!

because her mother is narcissa's and bellatrix's sister

Andromeda Tonks (nymphadora's mom) is sister to Narcissa and Bellatrix. So that would make Draco and Tonks cousins. :)

i think they r all cousins

Because Andromeda,Narcissa and Bellatrix are sisters.So Tonks is the daughter of Andromeda and that makes her Bellatrix and Narcissa's niece and Draco's first cousine.