"My sister is tall"
Translation:Dada yangu ni mrefu
Because dada is an N/N noun. Most animate nouns follow M/Wa rules regardless of class, but because N/N nouns are the same in both singular and plural you'd be stuck with dada wangu meaning both "my sister" and "my sisters". So the possessive agreements from N/N class are retained for clarity.
Wangu is both singular and plural. Dada doesn't change for singular and plural. That would be confusing, as we would never know how many sisters there are. So we use the same possessives as N/N nouns (which is the class dada belongs to anyway) to avoid confusion. It doesn't matter if it is singular or plural, it will always use N/N possessives.