Depending on the use year of the contract from which the points came--you may or may not have a problem if you cancel by 30 days prior to the trip. If you cancel within 30 days, the points go into "holding." There are significant restrictions on holding points and it's very possible they will be lost.
If you cancel more than 30 days out, the points will be returned to the current use year...if the contract's use year is more than 4 months out, they can be banked into next year. That is if the points were current year points. If they were borrowed or banked points, they'd have to be used by the end of the use year, or they'll be lost.
It's easier to understand with an example.
First, to avoid really difficult restrictions, it'd be best if you make your cancellation decision more than 30 days from the start of your trip.
If your family members' contract has a use year of December, February, April or June, the contract owner can bank the points and they'll be available to use next use year. If however the points are from a contract with a September or October use year, it's past the banking window and they'll have to be used prior to the end of the use year or they'll be lost. If your reservation was made with points that had already been banked, or were borrowed, they'll have to be used by the end of the current use year.
Pretty confusing if you're not used to these rules. Basically, what you need to know for us to tell you the consequences of you having to cancel is whether your reservation was made with banked, current or borrowed points; and the use year of the contract the points came from...And you need to cancel, if you're going to cancel, more than 30 days prior to the start of your trip or your family will be stuck with the holding points restrictions--and they are very tough and could well result in the points being completely lost...
Hope this helps, at least a bit.