It is Disney's policy that you can not use the pool if you are not a registered guest. I am pretty sure that it is standard in the hotel industry to not allow those who are not registered guests to use the amenities. Can you imagine if everyone did that, just invited those who are not staying at the hotel to use the pool? Do people go to their local Marriott and use the pool when they are not a registered guest? I find that many have this strange mentality when it comes to Disney that they somehow "owe" people the use of their product for free. I am sorry OP, but your sister and niece should not use the pool unless they are registered guests. If you are not at max occupancy for your room, you can go to the front desk and add them.