Hi Jake, Yes, you're right. I've copied the note below from the "People" skill:
Note: The exceptions to this [possessive pronoun prefix] rule are certain kinship nouns that come from Arabic: baba, mama, bibi, babu, shangazi, binamu, rafiki, dada, kaka, etc. These nouns take the y- agreement in the singular and the z- agreement in the plural.
Accordingly, "my sister" will be "dada yangu" whereas "my sisters" will be "dada zangu."