Hi! Thanks for joining MacInsiders. I wasn't in Engineering, but as far as I am aware there isn't different admission levels for coop, as long as you get accepted into Engineering.
If you want to apply early your sister can add it on through OUAC. Or you can add it on later once you're at McMaster right up until your final year of studies. It's flexible.
If your sister is going into B.Tech then it's already added in.
See this page for details about coop for prospective students:
http://www.eng.mcmaster.ca/eccs/Coop...tudents. html