Hello
The spouse/s who are not inside Canada need to fill IMM5604.
SInce you both are PRs and your wife is outside Canada, she CAN legally and technically become a co-signor but i would suggest to skip that and instead you being in Canada(assuming that you are inside Canada) should be sponsoring. No need of her becoming a co-signor. There are caselaws decided at IRB for what i said above regarding your wife being a co-signor. I practice law here in Canada and awaiting my Child PR sponsorship as well. Though nothing mentioned here is a legal advice.
Hope, it helps.
Thanks