"How many sisters do you have?"

Translation: Una dada wangapi?

[Jeremyjcy2k]

My Kenyan girlfriend says that there is no need to use the word have in this sentence - the have is understood.
Just for all our information.

[Everlighta]

Could anyone please explain wangapi instead of ngapi or zingapi? We say dada yangu/zangu, but dada wangapi, it confuses me.