I took my 12 yr old neice to Orlando last year which i paid for. My sister only had to get my neice's spending money together, and she only got that the day before we left for the airport. Then had the cheek to TELL me if my neice rang out of money I should sub her the cash while we were out there.
I told her to give my neice some english money to change at the airport, as i'm not going to pay for her spending money as well (you got to be cruel sometimes). She eventually gave my neice an extra 75 quid.
My niece's quite good with her money and did'nt go mad over there, spent most of her time swimming and not shopping

- she had plenty of cash left when we came back - so at the airport i told her to spend the lot before we went home.
So there was no DOLLARS or POUNDS left for sister

.
I'm assuming your holiday must be already paid in full if you only have 3 wks left. I would call your nephew parents and tell them that the full payment & insurance is due by say friday/saturday or you will lose all money you have already paid. What i did with the permission letter was type it out myself and got my sister to sign ( i was never ask to show it).
I would also get an idea about how much spending money they intend to give your nephew, as you may end up paying for this as well. The Problem is that because your the auntie they don't feel obliged to pay up. (Thank goodness not all family members are like this

).