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

her mother Andromeda is Narcissa's and Bellatrix's sister => ニンファドーラ・トンクス is their niece!

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

Andromeda ニンファドーラ・トンクス (nymphadora's mom) is sister to Narcissa and Bellatrix. So that would make Draco and ニンファドーラ・トンクス cousins. :)

i think they r all cousins

Because Andromeda,Narcissa and Bellatrix are sisters.So ニンファドーラ・トンクス is the daughter of Andromeda and that makes her Bellatrix and Narcissa's niece and Draco's first cousine.