niece    (daughter of sister, if a girl; daughter of brother, if boy) umukōbwacu, umukōbwanyu, umukōbwabo
(daughter of brother, if one is a girl) umusēngezana
(daughter of one's sister if one is a boy) umwīshwa
En-En dictionary 
nephew    (son of brother) umuhūngwacu, umusēngezana
(nephew or niece, child of one's sister if one is a boy) umwīshwa
En-En dictionary 
 niece found in: Kirundi II (Grammar)
chapter 53   Nouns of Relationship