Well, there's a couple ways you could do it depending on your goals:
Scenario #1: Staying at the Polynesian spending 250 points per year, book during at 7-11 months.
For this case, you need all 250 points to be Polynesian points. To accomplish this, you could buy 125 Poly points and your parents could buy 125 Poly points. Then, you can either transfer points from your parents contract to your contract every year and you book, or you could bank/borrow every other year on your contract and your parents could bank and borrow every other year on their contract.
For example, for 2016, you use your 125 points and borrow your 125 points from 2017 and spend all 250 points in 2016. Your parents bank their 2016 points to 2017 and then use all 250 points to book the 2017 trip. Then in 2018 you use your 2018 points and borrow from 2019 to book 2018, and so on.
Scenario #2: Staying at the Poly using 250 points every other year and at another resort (say BLT) every other year 250 points booking at 7-11 months.
To accomplish this, you buy 125 Poly points and your parents buy 125 BLT points, and use the same banking/borrowing trick as before, except since your parents now own BLT, you would have to stay at BLT every other year.
If you wanted to stay at a different resort, your parents can still transfer the BLT points to you, but you could only pool them together at 7 months.