I think you are being unreasonable. I say this from the perspective of the Aunt. I love my nieces and I am very, very close to my sisters (ther mother’s) however I would not agree to having them stay for up to 2 years, rent free (that’s how you OP reads) and to them treating m6 house as though it was their own, or the way the lived at home (eg: friends over at all hours, overnight guests, me doing more chores, cleaning etc after them, and generally feeling more restricted in my own entertaining with friends etc in my own home).
I work incredibly hard, and I treasure my time at home.
What I would do (and only as I am very close to my sisters and nieces) is offer for them to stay for up to 2-3 months, at nominal rent and with clearly defined house rules eg: cooking one night a week, no overnight guests, no friends over unless text beforehand, does her own cleaning, laundry etc of the spaces she uses.
Even then, it would be a big imposition for me, and I would feel pretty saintly for doing it and putting a lot of my life on hold to do it. I would also worry about the potential for it to go wrong and to result in a fall out with my niece (or more importantly to me - my sister). Frankly, MY life is busy enough, I try and do whatever possible to minimise my home/family life being difficult.
I would, however, love to see her for coffee/lunch, little shopping trips and shout her food and/or gift her some money when I saw her at those times, as my relatives occasionally did for me.