No, that's not going to work. As PPs have said, the max occupancy of the room is 4.
Even if you weren't over the limit, there's nowhere for them TO sleep. There are two queen beds in the room, no daybed or anything like that, so they'd be sleeping on the carpet. And the resort will not provide you with any additional sheets, blankets or pillows. Not to mention, six people sharing one bathroom wouldn't be optimal.
A one-bedroom won't work, either, the max occupancy of those at OKW is 5.
If you do want your sister and niece to stay with you for part of your vacation, I'd suggest going back to the member you rented from and ask if they'd be willing to try to change your reservation to a two-bedroom villa for the nights they'd be staying with you. It would be roughly twice the cost of the studio for those nights.