It’s always nice to see people being generous with their points and using them to help others!
With Velocity you can easily transfer your points between you and your sister, even if you’re not at the same address. Log into your Velocity account, go to “My Velocity”, then “Link & Transfer”, then “Family Points Transfer”. You can then make the one booking for your parents via Velocity, using the points you’ve transferred/pooled.
However, if you can’t find availability on Velocity and/or you want/need to use KrisFlyer points, things become a little more complex. To make the booking through KrisFlyer, you need to add your parents to your group of “Redemption Nominees” on your (or your sister’s) KrisFlyer account. Even if the four of you are in the same Redemption Nominees group, I really don’t think it’s possible to use two people’s points on the one booking, but I might be wrong on that. The easiest way to get an answer is to call or email KrisFlyer and ask. But you can’t transfer KrisFlyer points to someone else, in the way that you can with Velocity points.
Alternatively, you could transfer your Velocity points into the one account as detailed above, then transfer those Velocity points into the one KrisFlyer account, and make the booking using the points in that KrisFlyer account. Just be aware that Velocity points transfer into KrisFlyer at a 1.55:1 transfer rate (meaning that effectively you’ll lose about 35% of your Velocity points in the process).
The bottom line is that booking through Velocity will be much easier – if you can find availability. Otherwise you’ll need to use KrisFlyer. Let us know how you go!