I think the gesture is lovely, but I think there are logistics to Aulani that do not exist with other Disney destinations. We gave my nieces their Disney trip as a wedding gift. They had been planning a trip with us, so we paid for all of their expenses other than airfare and personal spending.
For your circumstance, in terms of what to do, I think for a honeymoon less is more, so that part should be bonus IMO. I have never had the good fortune to visit Hawaii, but have been told that this vacation is wasted on honeymoons. LOL! , but the time it takes to get there as well as the cost of the airfare may be problematic.
Personally, I would offer the trip as a gift, and detail what that offer includes. If you have points that you are using, I would offer them at another DVC destination where they cover the stay, and then let your niece decide.
In my case, my nieces had reached out to us to plan a DIsney trip with them, so offering it as a wedding gift was easy. We knew they wanted to go, and we knew we were going to pay for all of it. They did not know this, we had to surprise them with the balance paid off or they would have not accepted it. What we did need to do was prepare them for the cost of OOP expenses. Beverages, gratuities, etc all seem to escalate once you are trapped in a Disney resort, and I imagine Aulani is no different.
When you present them with your gift, I suggest you have a budget of what is included, but also what the basic stay will cost them, from airfare, to food and tips, and travel while there. So if board is included, what does that mean? Breakfast or every meal? Will that include gratuities, or should they budget for them? What is included in an Aulani stay and what it would cost to venture out?
FWIW I think the offer is lovely, and whether or not your niece can afford the air, I bet this offer will be a welcomed and treasured gift.