"This is Emilian's grandmother"
Translation:Huyu ni bibi yake Emilian
I understand your question. Yes, i agree it should be bibi ya emilian. However It isnt wa but ya. Family nouns like mama, baba, kaka, dada, etc are n/n or i/zi even though they are people.
Even though from English "bibi ya Emilian" seems more intuitive, "Bibi yake Emilian" is used more commonly.
The N- gender (Classes 9 / 10) nouns: When the noun it refers to is a person, the possessive adjective takes agreements of the N-gender, instead of the usual M-/WA- gender agreements, which makes it possible to distinguish between the singular and the plural.