Harry,
They should show you receipts of what they paid so that you know exactly what you need to pay for. You should insist that she shows these to you. You can calculate the fees for immigration using this fee calculator from CIC here: https://services3.cic.gc.ca/efee/efee.do I believe that the fees your sister would have had to pay would be $2230 in total:
* sponsorship application $75
* principal applicant $475
* family member of principal applicant over 22 (your mom) $550 --as your mom passed away, I'm not sure if any of this fee is re-imbursed. My guess is that it wouldn't be, but you would have to check.
* family member of principal applicant under 22 (you) $150
* right of permanent residence fee $490 X2 = $980
There is no tax on this. If your sister did not use a lawyer, then the immigration fees to CIC would have been all that she paid for. If your sister also paid for a lawyer then as mitamata said, there is no way to tell what the fees for this would have been. Again, you need to ask your sister to show you receipts. Good luck.