Nyakyusa is registered as M.31 in the list of Bantu languages. It is the only language in group M.30. Group M is put together more on a geographical than on a linguistic basis and one should not expect too many similarities within the group. Nyakyusa is one of the relatively few Bantu languages that doesn't make use of tones, but it also differs substantially from surrounding languages in terms of vocabulary. A monolingual speaker of Nyakyusa can not understand any other Bantu language. The "tree-structure" for Nyakyusa in the Niger-Cordofanian family: Niger-Cordofanian Niger-Congo Benue-Congo Bantoid M31 Nyakyusa The languages in group M are (Guthrie 1967): M 10 M 20 M 30 M 40 M 50 M 60 1 Pimbwe Wanda Nyakyusa Taabwa Biisa Lenje 2 Rungwa Mwanga Bemba Lala Soli 3 Fipa Nyiha Swaka Ila 4 Rungu Malila Lamba Tonga 5 Mambwe Safwa Seba 6 Iwa 7 Tembo Based on newer research with comparative wordlists it seems that the languages which come closest to Nyakyusa are the languages along the Zambian border.
(excerpt from the Nyakyusa dictionary)