I am currently indexing a will. Should I just index the name of the primary person i.e. the person whose will this is, or is there a way to add the relatives that are mentioned?
Follow the project instructions. Keep in mind that an index is not a transcription: what you're creating is a finding aid for the data, not a computerized version of (your reading of) the data.
The instruction is under What to Remember About this Project. If you close the PI's, you can always open them by clicking on the link at the bottom of your entry form. The Australia Will and Probate project has lots of pages that will be marked, No, No Extractable Data since you only index on the first page of the will. If the will has a codicil, then that date is the one you index with the will.