Where have you seen dada listed as belonging to 5/6?

Kinship names (other than mwana, mume, mke) are generally 9/10 and for the possessives they use the y- and z- forms to indicate plural and singular.

mama yangu = my mother
mama zangu = my mothers

baba yangu = my father
baba zangu = my fathers

kaka yangu = my brother
kaka zangu = my brothers

dada yangu = my sister
dada zangu = my sisters

Same with bibi/nyanya and babu, ndugu, shemeji and a bunch of others.

These words sometimes get a ma- in the plural which seems to be a non-standard thing, such as "rafiki", which is commonly:

rafiki yangu = my friend
marafiki zangu / rafiki zangu = my friends

But that's pretty rare with dada.