..which is about right, depending on the clinic. 18K sounds fairly average for a fresh cycle to me, as long it as includes everything for both you and your donor. Some clinics already mentioned will be MUCH cheaper, especially if they offer shared risk cycle or guaranteed programs, but since you're using your sister, you probably wouldn't want or be able do this type of program.
I used my cousin, and was in a similar situation. We chose not to do any of the guarantee programs because she only wanted to donate once, which was completely understandable to me. It's a big chance and using a family member comes with A LOT of issues, but it can be a beautiful thing. I've heard a fair amount of negative reaction to using family members on the board, but I think most people are just making the point that you need to have your eyes wide open to all the potential problems. Make sure your clinic uses a psychologist or some type of therapist familiar with all the issues and make sure you have both individual and joint counseling sessions. Legal representation will also be necessary.
Let me know if you have any specific questions. Having gone through this with my cousin and maintained a great relationship, I'd be happy to talk with you more about it.