The family with the August use year can reserve a November 2018 vacation by borrowing their 2019 points. Have them reserve half the nights and you reserve the other half. (If you are staying an odd number of nights, you'll have to work out compensation for that last night between yourselves). Each of you will reserve the nights when the window for your part of the vacation opens. MS can note on both reservations that you want to stay in the same room - all of your names will be on both reservations to facilitate ADRs and FP+ reservations.
AFAIK, transferred points cannot be borrowed (only banked). Since transfers normally go same use year to same use year, they would have to transfer their 2019 points to your 2019 use year and so a transfer won't work. (They can't borrow their points into 2018 and then transfer the points, either).
That said, you should check with MS to see if there is a way to transfer their 2019 points to your 2018 use year. If that can be done (be sure to tell MS what dates for which you want to use the transferred points before you do the transfer), have MS make the reservation on that same call.
Good luck!