What should I do if my former advisor wants to use my research project data (including dissertation) for the current PhD student's dissertation?
It would be good if you can find a way for this to work to everyone's benefit.
I'd suggest that you start with your advisor and express your concerns and get some assurance that the other student won't be stepping on your research. You should be able to get a great acknowledgement in work of the other student. And your own work might well be eligible for citation under these circumstances.
But also express your need for the time for your own work and that  you can't prioritize helping the other student at this time.
Once you have some agreement from  your advisor, initiate a three way conversation in which everything is made clear. You'll help when can, but have other priorities that must come first. But from your description, it sounds like it is possible for everyone to win as long as the others respect your limits.