It can be yours if you are going to be the representative. Otherwise principal applicant's.Step1 - While creating account on Permanent Residence Portal (https://prson-srpel.apps.cic.gc.ca/en/login)- Email should be of Principal Applicatnt (my mom) or Sponser (thats me) ?
If you are going to be their representative, You will still use the Permanent Residence Portal (https://prson-srpel.apps.cic.gc.ca/en/login). Just need to add USE OF REPRESENTATIVE FORM.
Representative Portal (https://prson-srpel.apps.cic.gc.ca/en/rep/login) is only for paid representatives.