At my transplant center, they claim that if a donor ends up needing a kidney, that they will immediately go to the top of the transplant list. I would ask about that - is it true? Is this a national rule or center-specific?
I would also talk to your cousin about your worries, and see if she is equally concerned about these issues. It is difficult to make the idea of dialysis real to someone who has never experienced it. Personally, I hate it when my husband (my future donor) answers my direct questions about dialysis with vagaries. I want him to tell me explicitly how he will manage if this becomes a reality.
I like Richard's advice about not getting too far ahead of yourselves with worry. Your cousin can back out at any time, but so can you. If she goes through the tests and is approved, you can still tell the transplant center that you are not comfortable with this option and want to wait for a cadaver/stay on dialysis/go with another donor.
Please do be careful about discussing this at the psychosocial eval. I really think this is the sort of thing that is best discussed with your own private therapist with no affiliation to the hospital. Those social workers and psychiatrists are not your therapists, they are there first and foremost to judge you, which rather conflicts with getting anything useful out of them. If you just want facts about risks, then speak to the transplant doctors, but again, do not kid yourself into thinking that this is an objective opinion. Actual response I received from a transplant surgeon when I asked him what are the risks of donating a kidney: "There are none."
I think for the psychological eval, it is best to not seem conflicted about your decision. The only questions I remember being asked about the donor were about whether we had a strong relationship or not. If they did not ask me a specific question, I was not going to give them any more information than they requested. (Can you tell I think this part of the eval is idiotic?)
The risks to the donor do seem to be quite small, and the overall impact on life minimal. I have mentioned before that my original donor is still alive after 33 years, still fine, and decided to donate bone marrow to a stranger 20 years after donating a kidney to me. I wouldn't be in the least surprised if he were to donate a lung or part of his liver in the future, and after he passes away, I have no doubt that it will be "take what you can use, and give the rest to medical science." Some people are like that....
Good luck speaking with your cousin! Keep us updated!