Just a very basic question before you think about renting or transferring such a large number of points...can you borrow from your next Use Year?
If not, and this is just me, I honestly would tell family that I cannot do it this year. We have, so far, invited extended family with us for all of our WDW trips, but we don't have the points to have them with us the WHOLE time (and they work anyway, and are semi-local, so they don't mind a shorter stay), and once we run out of points to bank and borrow (this trip in Feb is it for awhile!) we're not going to rent or transfer. It's one thing to use the points you already have...it's another thing in our opinion to put money out of pocket to use in that way.
But that's just us.
Anyway, make sure you can't borrow from the next year in your own account to make up the difference before thinking of those things.