The same thing that makes the two sisters be together and to complete the nobility of Rias, just put an Oc from another anime (that is of the cat race xD).
Yasaka in the harem? Please do not put Kunou in the harem in almost all the dxd fanfictions that I read always make Kunou like her little sister, her little sister, now it makes me strange to see her in a harem even though they increase her age.